chore: add vendor directory

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diff --git a/vendor/multimap/src/lib.rs b/vendor/multimap/src/lib.rs
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+#![forbid(unsafe_code)]
+// Copyright (c) 2016 multimap developers
+//
+// Licensed under the Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0> or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. All files in the project carrying such notice may not be copied,
+// modified, or distributed except according to those terms.
+
+//! A map implementation which allows storing multiple values per key.
+//!
+//! The interface is roughly based on std::collections::HashMap, but is changed
+//! and extended to accomodate the multi-value use case. In fact, MultiMap is
+//! implemented mostly as a thin wrapper around std::collections::HashMap and
+//! stores its values as a std::Vec per key.
+//!
+//! Values are guaranteed to be in insertion order as long as not manually
+//! changed. Keys are not ordered. Multiple idential key-value-pairs can exist
+//! in the MultiMap. A key can exist in the MultiMap with no associated value.
+//!
+//! # Examples
+//!
+//! ```
+//! use multimap::MultiMap;
+//!
+//! // create a new MultiMap. An explicit type signature can be omitted because of the
+//! // type inference.
+//! let mut queries = MultiMap::new();
+//!
+//! // insert some queries.
+//! queries.insert("urls", "http://rust-lang.org");
+//! queries.insert("urls", "http://mozilla.org");
+//! queries.insert("urls", "http://wikipedia.org");
+//! queries.insert("id", "42");
+//! queries.insert("name", "roger");
+//!
+//! // check if there's any urls.
+//! println!("Are there any urls in the multimap? {:?}.",
+//!     if queries.contains_key("urls") {"Yes"} else {"No"} );
+//!
+//! // get the first item in a key's vector.
+//! assert_eq!(queries.get("urls"), Some(&"http://rust-lang.org"));
+//!
+//! // get all the urls.
+//! assert_eq!(queries.get_vec("urls"),
+//!     Some(&vec!["http://rust-lang.org", "http://mozilla.org", "http://wikipedia.org"]));
+//!
+//! // iterate over all keys and the first value in the key's vector.
+//! for (key, value) in queries.iter() {
+//!     println!("key: {:?}, val: {:?}", key, value);
+//! }
+//!
+//! // iterate over all keys and the key's vector.
+//! for (key, values) in queries.iter_all() {
+//!     println!("key: {:?}, values: {:?}", key, values);
+//! }
+//!
+//! // the different methods for getting value(s) from the multimap.
+//! let mut map = MultiMap::new();
+//!
+//! map.insert("key1", 42);
+//! map.insert("key1", 1337);
+//!
+//! assert_eq!(map["key1"], 42);
+//! assert_eq!(map.get("key1"), Some(&42));
+//! assert_eq!(map.get_vec("key1"), Some(&vec![42, 1337]));
+//! ```
+
+use std::borrow::Borrow;
+use std::collections::hash_map::{IntoIter, Keys, RandomState};
+use std::collections::HashMap;
+use std::fmt::{self, Debug};
+use std::hash::{BuildHasher, Hash};
+use std::iter::{FromIterator, IntoIterator, Iterator};
+use std::ops::Index;
+
+pub use std::collections::hash_map::Iter as IterAll;
+pub use std::collections::hash_map::IterMut as IterAllMut;
+
+pub use entry::{Entry, OccupiedEntry, VacantEntry};
+
+mod entry;
+
+#[cfg(feature = "serde_impl")]
+pub mod serde;
+
+#[derive(Clone)]
+pub struct MultiMap<K, V, S = RandomState> {
+    inner: HashMap<K, Vec<V>, S>,
+}
+
+impl<K, V> MultiMap<K, V>
+where
+    K: Eq + Hash,
+{
+    /// Creates an empty MultiMap
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map: MultiMap<&str, isize> = MultiMap::new();
+    /// ```
+    pub fn new() -> MultiMap<K, V> {
+        MultiMap {
+            inner: HashMap::new(),
+        }
+    }
+
+    /// Creates an empty multimap with the given initial capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map: MultiMap<&str, isize> = MultiMap::with_capacity(20);
+    /// ```
+    pub fn with_capacity(capacity: usize) -> MultiMap<K, V> {
+        MultiMap {
+            inner: HashMap::with_capacity(capacity),
+        }
+    }
+}
+
+impl<K, V, S> MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    /// Creates an empty MultiMap which will use the given hash builder to hash keys.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    /// use std::collections::hash_map::RandomState;
+    ///
+    /// let s = RandomState::new();
+    /// let mut map: MultiMap<&str, isize> = MultiMap::with_hasher(s);
+    /// ```
+    pub fn with_hasher(hash_builder: S) -> MultiMap<K, V, S> {
+        MultiMap {
+            inner: HashMap::with_hasher(hash_builder),
+        }
+    }
+
+    /// Creates an empty MultiMap with the given intial capacity and hash builder.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    /// use std::collections::hash_map::RandomState;
+    ///
+    /// let s = RandomState::new();
+    /// let mut map: MultiMap<&str, isize> = MultiMap::with_capacity_and_hasher(20, s);
+    /// ```
+    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> MultiMap<K, V, S> {
+        MultiMap {
+            inner: HashMap::with_capacity_and_hasher(capacity, hash_builder),
+        }
+    }
+
+    /// Inserts a key-value pair into the multimap. If the key does exist in
+    /// the map then the value is pushed to that key's vector. If the key doesn't
+    /// exist in the map a new vector with the given value is inserted.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert("key", 42);
+    /// ```
+    pub fn insert(&mut self, k: K, v: V) {
+        match self.entry(k) {
+            Entry::Occupied(mut entry) => {
+                entry.get_vec_mut().push(v);
+            }
+            Entry::Vacant(entry) => {
+                entry.insert_vec(vec![v]);
+            }
+        }
+    }
+
+    /// Inserts multiple key-value pairs into the multimap. If the key does exist in
+    /// the map then the values are extended into that key's vector. If the key
+    /// doesn't exist in the map a new vector collected from the given values is inserted.
+    ///
+    /// This may be more efficient than inserting values independently.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::<&str, &usize>::new();
+    /// map.insert_many("key", &[42, 43]);
+    /// ```
+    pub fn insert_many<I: IntoIterator<Item = V>>(&mut self, k: K, v: I) {
+        match self.entry(k) {
+            Entry::Occupied(mut entry) => {
+                entry.get_vec_mut().extend(v);
+            }
+            Entry::Vacant(entry) => {
+                entry.insert_vec(v.into_iter().collect::<Vec<_>>());
+            }
+        }
+    }
+
+    /// Inserts multiple key-value pairs into the multimap. If the key does exist in
+    /// the map then the values are extended into that key's vector. If the key
+    /// doesn't exist in the map a new vector collected from the given values is inserted.
+    ///
+    /// This may be more efficient than inserting values independently.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::<&str, usize>::new();
+    /// map.insert_many_from_slice("key", &[42, 43]);
+    /// ```
+    pub fn insert_many_from_slice(&mut self, k: K, v: &[V])
+    where
+        V: Clone,
+    {
+        match self.entry(k) {
+            Entry::Occupied(mut entry) => {
+                entry.get_vec_mut().extend_from_slice(v);
+            }
+            Entry::Vacant(entry) => {
+                entry.insert_vec(v.to_vec());
+            }
+        }
+    }
+
+    /// Returns true if the map contains a value for the specified key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// assert_eq!(map.contains_key(&1), true);
+    /// assert_eq!(map.contains_key(&2), false);
+    /// ```
+    pub fn contains_key<Q>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.contains_key(k)
+    }
+
+    /// Returns the number of unique keys in the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(2, 1337);
+    /// map.insert(2, 31337);
+    /// assert_eq!(map.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    /// Removes a key from the map, returning the vector of values at
+    /// the key if the key was previously in the map.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// assert_eq!(map.remove(&1), Some(vec![42, 1337]));
+    /// assert_eq!(map.remove(&1), None);
+    /// ```
+    pub fn remove<Q>(&mut self, k: &Q) -> Option<Vec<V>>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.remove(k)
+    }
+
+    /// Returns a reference to the first item in the vector corresponding to
+    /// the key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// assert_eq!(map.get(&1), Some(&42));
+    /// ```
+    pub fn get<Q>(&self, k: &Q) -> Option<&V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.get(k)?.first()
+    }
+
+    /// Returns a mutable reference to the first item in the vector corresponding to
+    /// the key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// if let Some(v) = map.get_mut(&1) {
+    ///     *v = 99;
+    /// }
+    /// assert_eq!(map[&1], 99);
+    /// ```
+    pub fn get_mut<Q>(&mut self, k: &Q) -> Option<&mut V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.get_mut(k)?.get_mut(0)
+    }
+
+    /// Returns a reference to the vector corresponding to the key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// assert_eq!(map.get_vec(&1), Some(&vec![42, 1337]));
+    /// ```
+    pub fn get_vec<Q>(&self, k: &Q) -> Option<&Vec<V>>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.get(k)
+    }
+
+    /// Returns a mutable reference to the vector corresponding to the key.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// if let Some(v) = map.get_vec_mut(&1) {
+    ///     (*v)[0] = 1991;
+    ///     (*v)[1] = 2332;
+    /// }
+    /// assert_eq!(map.get_vec(&1), Some(&vec![1991, 2332]));
+    /// ```
+    pub fn get_vec_mut<Q>(&mut self, k: &Q) -> Option<&mut Vec<V>>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        self.inner.get_mut(k)
+    }
+
+    /// Returns true if the key is multi-valued.
+    ///
+    /// The key may be any borrowed form of the map's key type, but Hash and Eq
+    /// on the borrowed form must match those for the key type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1, 42);
+    /// map.insert(1, 1337);
+    /// map.insert(2, 2332);
+    ///
+    /// assert_eq!(map.is_vec(&1), true);   // key is multi-valued
+    /// assert_eq!(map.is_vec(&2), false);  // key is single-valued
+    /// assert_eq!(map.is_vec(&3), false);  // key not in map
+    /// ```
+    pub fn is_vec<Q>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash + ?Sized,
+    {
+        match self.get_vec(k) {
+            Some(val) => val.len() > 1,
+            None => false,
+        }
+    }
+
+    /// Returns the number of elements the map can hold without reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let map: MultiMap<usize, usize> = MultiMap::new();
+    /// assert!(map.capacity() >= 0);
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
+    /// Returns true if the map contains no elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// assert!(map.is_empty());
+    /// map.insert(1,42);
+    /// assert!(!map.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.inner.is_empty()
+    }
+
+    /// Clears the map, removing all key-value pairs.
+    /// Keeps the allocated memory for reuse.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.clear();
+    /// assert!(map.is_empty());
+    /// ```
+    pub fn clear(&mut self) {
+        self.inner.clear();
+    }
+
+    /// An iterator visiting all keys in arbitrary order.
+    /// Iterator element type is &'a K.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(2,1337);
+    /// map.insert(4,1991);
+    ///
+    /// let mut keys: Vec<_> = map.keys().collect();
+    /// keys.sort();
+    /// assert_eq!(keys, [&1, &2, &4]);
+    /// ```
+    pub fn keys(&'_ self) -> Keys<'_, K, Vec<V>> {
+        self.inner.keys()
+    }
+
+    /// An iterator visiting pairs of each key and its first value in arbitrary order.
+    /// The iterator returns
+    /// a reference to the key and the first element in the corresponding key's vector.
+    /// Iterator element type is (&'a K, &'a V).
+    ///
+    /// See [`flat_iter`](Self::flat_iter)
+    /// for visiting all key-value pairs,
+    /// or [`iter_all`](Self::iter_all)
+    /// for visiting each key and its vector of values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// let mut pairs: Vec<_> = map.iter().collect();
+    /// pairs.sort_by_key(|p| p.0);
+    /// assert_eq!(pairs, [(&1, &42), (&3, &2332), (&4, &1991)]);
+    /// ```
+    pub fn iter(&self) -> Iter<K, V> {
+        Iter {
+            inner: self.inner.iter(),
+        }
+    }
+
+    /// A mutable iterator visiting pairs of each key and its first value
+    /// in arbitrary order. The iterator returns
+    /// a reference to the key and a mutable reference to the first element in the
+    /// corresponding key's vector. Iterator element type is (&'a K, &'a mut V).
+    ///
+    /// See [`flat_iter_mut`](Self::flat_iter_mut)
+    /// for visiting all key-value pairs,
+    /// or [`iter_all_mut`](Self::iter_all_mut)
+    /// for visiting each key and its vector of values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// for (_, value) in map.iter_mut() {
+    ///     *value *= *value;
+    /// }
+    ///
+    /// let mut pairs: Vec<_> = map.iter_mut().collect();
+    /// pairs.sort_by_key(|p| p.0);
+    /// assert_eq!(pairs, [(&1, &mut 1764), (&3, &mut 5438224), (&4, &mut 3964081)]);
+    /// ```
+    pub fn iter_mut(&mut self) -> IterMut<K, V> {
+        IterMut {
+            inner: self.inner.iter_mut(),
+        }
+    }
+
+    /// An iterator visiting all key-value pairs in arbitrary order. The iterator returns
+    /// a reference to the key and the corresponding key's vector.
+    /// Iterator element type is (&'a K, &'a V).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// let mut pairs: Vec<_> = map.iter_all().collect();
+    /// pairs.sort_by_key(|p| p.0);
+    /// assert_eq!(pairs, [(&1, &vec![42, 1337]), (&3, &vec![2332]), (&4, &vec![1991])]);
+    /// ```
+    pub fn iter_all(&self) -> IterAll<K, Vec<V>> {
+        self.inner.iter()
+    }
+
+    /// An iterator visiting all key-value pairs in arbitrary order. The iterator returns
+    /// a reference to the key and the corresponding key's vector.
+    /// Iterator element type is (&'a K, &'a V).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// for (key, values) in map.iter_all_mut() {
+    ///     for value in values.iter_mut() {
+    ///         *value = 99;
+    ///     }
+    /// }
+    ///
+    /// let mut pairs: Vec<_> = map.iter_all_mut().collect();
+    /// pairs.sort_by_key(|p| p.0);
+    /// assert_eq!(pairs, [(&1, &mut vec![99, 99]), (&3, &mut vec![99]), (&4, &mut vec![99])]);
+    /// ```
+    pub fn iter_all_mut(&mut self) -> IterAllMut<K, Vec<V>> {
+        self.inner.iter_mut()
+    }
+
+    /// An iterator visiting all key-value pairs in arbitrary order.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// let mut pairs: Vec<_> = map.flat_iter().collect();
+    /// pairs.sort();
+    /// assert_eq!(pairs, [(&1, &42), (&1, &1337), (&3, &2332), (&4, &1991)]);
+    /// ```
+    pub fn flat_iter(&self) -> impl Iterator<Item = (&K, &V)> {
+        self.iter_all()
+            .flat_map(|(k, v)| v.iter().map(move |i| (k, i)))
+    }
+
+    /// A mutable iterator visiting all key-value pairs in arbitrary order.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut map = MultiMap::new();
+    /// map.insert(1,42);
+    /// map.insert(1,1337);
+    /// map.insert(3,2332);
+    /// map.insert(4,1991);
+    ///
+    /// for (key, value) in map.flat_iter_mut() {
+    ///     *value *= key;
+    /// }
+    ///
+    /// let mut pairs: Vec<_> = map.flat_iter().collect();
+    /// pairs.sort();
+    /// assert_eq!(pairs, [(&1, &42), (&1, &1337), (&3, &6996), (&4, &7964)]);
+    /// ```
+    pub fn flat_iter_mut(&mut self) -> impl Iterator<Item = (&K, &mut V)> {
+        self.iter_all_mut()
+            .flat_map(|(k, v)| v.iter_mut().map(move |i| (k, i)))
+    }
+
+    /// Gets the specified key's corresponding entry in the map for in-place manipulation.
+    /// It's possible to both manipulate the vector and the 'value' (the first value in the
+    /// vector).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut m = MultiMap::new();
+    /// m.insert(1, 42);
+    ///
+    /// {
+    ///     let mut v = m.entry(1).or_insert(43);
+    ///     assert_eq!(v, &42);
+    ///     *v = 44;
+    /// }
+    /// assert_eq!(m.entry(2).or_insert(666), &666);
+    ///
+    /// {
+    ///     let mut v = m.entry(1).or_insert_vec(vec![43]);
+    ///     assert_eq!(v, &vec![44]);
+    ///     v.push(50);
+    /// }
+    /// assert_eq!(m.entry(2).or_insert_vec(vec![667]), &vec![666]);
+    ///
+    /// assert_eq!(m.get_vec(&1), Some(&vec![44, 50]));
+    /// ```
+    pub fn entry(&mut self, k: K) -> Entry<K, V> {
+        use std::collections::hash_map::Entry as HashMapEntry;
+        match self.inner.entry(k) {
+            HashMapEntry::Occupied(entry) => Entry::Occupied(OccupiedEntry { inner: entry }),
+            HashMapEntry::Vacant(entry) => Entry::Vacant(VacantEntry { inner: entry }),
+        }
+    }
+
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all pairs `(k, v)` such that `f(&k,&mut v)` returns `false`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use multimap::MultiMap;
+    ///
+    /// let mut m = MultiMap::new();
+    /// m.insert(1, 42);
+    /// m.insert(1, 99);
+    /// m.insert(2, 42);
+    /// m.retain(|&k, &v| { k == 1 && v == 42 });
+    /// assert_eq!(1, m.len());
+    /// assert_eq!(Some(&42), m.get(&1));
+    /// ```
+    pub fn retain<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&K, &V) -> bool,
+    {
+        for (key, vector) in &mut self.inner {
+            vector.retain(|value| f(key, value));
+        }
+        self.inner.retain(|_, v| !v.is_empty());
+    }
+}
+
+impl<K, V, S, Q> Index<&Q> for MultiMap<K, V, S>
+where
+    K: Eq + Hash + Borrow<Q>,
+    Q: Eq + Hash + ?Sized,
+    S: BuildHasher,
+{
+    type Output = V;
+
+    fn index(&self, index: &Q) -> &V {
+        self.inner
+            .get(index)
+            .expect("no entry found for key")
+            .first()
+            .expect("no value found for key")
+    }
+}
+
+impl<K, V, S> Debug for MultiMap<K, V, S>
+where
+    K: Eq + Hash + Debug,
+    V: Debug,
+    S: BuildHasher,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_map().entries(self.iter_all()).finish()
+    }
+}
+
+impl<K, V, S> PartialEq for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    V: PartialEq,
+    S: BuildHasher,
+{
+    fn eq(&self, other: &MultiMap<K, V, S>) -> bool {
+        if self.len() != other.len() {
+            return false;
+        }
+
+        self.iter_all()
+            .all(|(key, value)| other.get_vec(key).is_some_and(|v| *value == *v))
+    }
+}
+
+impl<K, V, S> Eq for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    V: Eq,
+    S: BuildHasher,
+{
+}
+
+impl<K, V, S> Default for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher + Default,
+{
+    fn default() -> MultiMap<K, V, S> {
+        MultiMap {
+            inner: Default::default(),
+        }
+    }
+}
+
+impl<K, V, S> FromIterator<(K, V)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher + Default,
+{
+    fn from_iter<T: IntoIterator<Item = (K, V)>>(iterable: T) -> MultiMap<K, V, S> {
+        let iter = iterable.into_iter();
+        let hint = iter.size_hint().0;
+
+        let mut multimap = MultiMap::with_capacity_and_hasher(hint, S::default());
+        for (k, v) in iter {
+            multimap.insert(k, v);
+        }
+
+        multimap
+    }
+}
+
+impl<K, V, S> FromIterator<(K, Vec<V>)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    V: Clone,
+    S: BuildHasher + Default,
+{
+    fn from_iter<T: IntoIterator<Item = (K, Vec<V>)>>(iterable: T) -> MultiMap<K, V, S> {
+        let iter = iterable.into_iter();
+        let hint = iter.size_hint().0;
+
+        let mut multimap = MultiMap::with_capacity_and_hasher(hint, S::default());
+        for (k, v) in iter {
+            multimap.insert_many_from_slice(k, &v[..])
+        }
+
+        multimap
+    }
+}
+
+impl<'a, K, V, S> IntoIterator for &'a MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    type Item = (&'a K, &'a Vec<V>);
+    type IntoIter = IterAll<'a, K, Vec<V>>;
+
+    fn into_iter(self) -> IterAll<'a, K, Vec<V>> {
+        self.iter_all()
+    }
+}
+
+impl<'a, K, V, S> IntoIterator for &'a mut MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    type Item = (&'a K, &'a mut Vec<V>);
+    type IntoIter = IterAllMut<'a, K, Vec<V>>;
+
+    fn into_iter(self) -> IterAllMut<'a, K, Vec<V>> {
+        self.inner.iter_mut()
+    }
+}
+
+impl<K, V, S> IntoIterator for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    type Item = (K, Vec<V>);
+    type IntoIter = IntoIter<K, Vec<V>>;
+
+    fn into_iter(self) -> IntoIter<K, Vec<V>> {
+        self.inner.into_iter()
+    }
+}
+
+impl<K, V, S> Extend<(K, V)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
+        for (k, v) in iter {
+            self.insert(k, v);
+        }
+    }
+}
+
+impl<'a, K, V, S> Extend<(&'a K, &'a V)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash + Copy,
+    V: Copy,
+    S: BuildHasher,
+{
+    fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T) {
+        self.extend(iter.into_iter().map(|(&key, &value)| (key, value)));
+    }
+}
+
+impl<K, V, S> Extend<(K, Vec<V>)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash,
+    S: BuildHasher,
+{
+    fn extend<T: IntoIterator<Item = (K, Vec<V>)>>(&mut self, iter: T) {
+        for (k, values) in iter {
+            match self.entry(k) {
+                Entry::Occupied(mut entry) => {
+                    entry.get_vec_mut().extend(values);
+                }
+                Entry::Vacant(entry) => {
+                    entry.insert_vec(values);
+                }
+            }
+        }
+    }
+}
+
+impl<'a, K, V, S> Extend<(&'a K, &'a Vec<V>)> for MultiMap<K, V, S>
+where
+    K: Eq + Hash + Copy,
+    V: Copy,
+    S: BuildHasher,
+{
+    fn extend<T: IntoIterator<Item = (&'a K, &'a Vec<V>)>>(&mut self, iter: T) {
+        self.extend(
+            iter.into_iter()
+                .map(|(&key, values)| (key, values.to_owned())),
+        );
+    }
+}
+
+#[derive(Clone)]
+pub struct Iter<'a, K: 'a, V: 'a> {
+    inner: IterAll<'a, K, Vec<V>>,
+}
+
+impl<'a, K, V> Iterator for Iter<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<(&'a K, &'a V)> {
+        let (k, v) = self.inner.next()?;
+        let v = v.first()?;
+        Some((k, v))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<K, V> ExactSizeIterator for Iter<'_, K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+pub struct IterMut<'a, K: 'a, V: 'a> {
+    inner: IterAllMut<'a, K, Vec<V>>,
+}
+
+impl<'a, K, V> Iterator for IterMut<'a, K, V> {
+    type Item = (&'a K, &'a mut V);
+
+    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
+        let (k, v) = self.inner.next()?;
+        let v = v.first_mut()?;
+        Some((k, v))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<K, V> ExactSizeIterator for IterMut<'_, K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+#[macro_export]
+/// Create a `MultiMap` from a list of key value pairs
+///
+/// ## Example
+///
+/// ```
+/// # use multimap::MultiMap;
+/// #[macro_use] extern crate multimap;
+/// # fn main(){
+///
+/// let map = multimap!(
+///     "dog" => "husky",
+///     "dog" => "retreaver",
+///     "dog" => "shiba inu",
+///     "cat" => "cat"
+///     );
+/// # }
+///
+/// ```
+macro_rules! multimap{
+    (@replace_with_unit $_t:tt) => { () };
+    (@count $($key:expr),*) => { <[()]>::len(&[$($crate::multimap! { @replace_with_unit $key }),*]) };
+
+    ($($key:expr => $value:expr),* $(,)?)=>{
+        {
+            let mut map = $crate::MultiMap::with_capacity($crate::multimap! { @count $($key),* });
+            $(
+                map.insert($key,$value);
+             )*
+            map
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashMap;
+    use std::iter::FromIterator;
+
+    use super::*;
+
+    #[test]
+    fn create() {
+        let _: MultiMap<usize, usize> = MultiMap {
+            inner: HashMap::new(),
+        };
+    }
+
+    #[test]
+    fn new() {
+        let _: MultiMap<usize, usize> = MultiMap::new();
+    }
+
+    #[test]
+    fn with_capacity() {
+        let _: MultiMap<usize, usize> = MultiMap::with_capacity(20);
+    }
+
+    #[test]
+    fn insert() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 3);
+    }
+
+    #[test]
+    fn insert_identical() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 42);
+        assert_eq!(m.get_vec(&1), Some(&vec![42, 42]));
+    }
+
+    #[test]
+    fn insert_many() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert_many(1, vec![3, 4]);
+        assert_eq!(Some(&vec![3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    fn insert_many_again() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 2);
+        m.insert_many(1, vec![3, 4]);
+        assert_eq!(Some(&vec![2, 3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    fn insert_many_overlap() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert_many(1, vec![2, 3]);
+        m.insert_many(1, vec![3, 4]);
+        assert_eq!(Some(&vec![2, 3, 3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    fn insert_many_from_slice() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert_many_from_slice(1, &[3, 4]);
+        assert_eq!(Some(&vec![3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    fn insert_many_from_slice_again() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 2);
+        m.insert_many_from_slice(1, &[3, 4]);
+        assert_eq!(Some(&vec![2, 3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    fn insert_existing() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 3);
+        m.insert(1, 4);
+        assert_eq!(Some(&vec![3, 4]), m.get_vec(&1));
+    }
+
+    #[test]
+    #[should_panic(expected = "no entry found for key")]
+    fn index_no_entry() {
+        let m: MultiMap<usize, usize> = MultiMap::new();
+        let _ = &m[&1];
+    }
+
+    #[test]
+    fn index() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 41);
+        m.insert(2, 42);
+        m.insert(3, 43);
+        let values = m[&2];
+        assert_eq!(values, 42);
+    }
+
+    #[test]
+    #[should_panic(expected = "no value found for key")]
+    fn index_empty_vec() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.get_vec_mut(&1).unwrap().clear();
+        let values = m[&1];
+        assert_eq!(values, 42);
+    }
+
+    #[test]
+    fn contains_key_true() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        assert!(m.contains_key(&1));
+    }
+
+    #[test]
+    fn contains_key_false() {
+        let m: MultiMap<usize, usize> = MultiMap::new();
+        assert!(!m.contains_key(&1));
+    }
+
+    #[test]
+    fn len() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(2, 1337);
+        m.insert(3, 99);
+        assert_eq!(m.len(), 3);
+    }
+
+    #[test]
+    fn remove_not_present() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        let v = m.remove(&1);
+        assert_eq!(v, None);
+    }
+
+    #[test]
+    fn remove_present() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        let v = m.remove(&1);
+        assert_eq!(v, Some(vec![42]));
+    }
+
+    #[test]
+    fn get_not_present() {
+        let m: MultiMap<usize, usize> = MultiMap::new();
+        assert_eq!(m.get(&1), None);
+    }
+
+    #[test]
+    fn get_present() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        assert_eq!(m.get(&1), Some(&42));
+    }
+
+    #[test]
+    fn get_empty() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.get_vec_mut(&1).and_then(Vec::pop);
+        assert_eq!(m.get(&1), None);
+    }
+
+    #[test]
+    fn get_vec_not_present() {
+        let m: MultiMap<usize, usize> = MultiMap::new();
+        assert_eq!(m.get_vec(&1), None);
+    }
+
+    #[test]
+    fn get_vec_present() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 1337);
+        assert_eq!(m.get_vec(&1), Some(&vec![42, 1337]));
+    }
+
+    #[test]
+    fn capacity() {
+        let m: MultiMap<usize, usize> = MultiMap::with_capacity(20);
+        assert!(m.capacity() >= 20);
+    }
+
+    #[test]
+    fn is_empty_true() {
+        let m: MultiMap<usize, usize> = MultiMap::new();
+        assert!(m.is_empty());
+    }
+
+    #[test]
+    fn is_empty_false() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        assert!(!m.is_empty());
+    }
+
+    #[test]
+    fn clear() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.clear();
+        assert!(m.is_empty());
+    }
+
+    #[test]
+    fn get_mut() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        if let Some(v) = m.get_mut(&1) {
+            *v = 1337;
+        }
+        assert_eq!(m[&1], 1337)
+    }
+
+    #[test]
+    fn get_vec_mut() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 1337);
+        if let Some(v) = m.get_vec_mut(&1) {
+            (*v)[0] = 5;
+            (*v)[1] = 10;
+        }
+        assert_eq!(m.get_vec(&1), Some(&vec![5, 10]))
+    }
+
+    #[test]
+    fn get_mut_empty() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.get_vec_mut(&1).and_then(Vec::pop);
+        assert_eq!(m.get_mut(&1), None);
+    }
+
+    #[test]
+    fn keys() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(2, 42);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys: Vec<_> = m.keys().cloned().collect();
+        assert_eq!(keys.len(), 4);
+        assert!(keys.contains(&1));
+        assert!(keys.contains(&2));
+        assert!(keys.contains(&4));
+        assert!(keys.contains(&8));
+    }
+
+    #[test]
+    fn iter() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 42);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        assert!(m.iter().all(|(_, &v)| v == 42));
+
+        let mut iter = m.iter();
+
+        for _ in iter.by_ref().take(2) {}
+
+        assert_eq!(iter.len(), 1);
+    }
+
+    #[test]
+    fn iter_empty_vec() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(42, 42);
+        m.get_vec_mut(&42).unwrap().clear();
+
+        assert!(m.iter().next().is_none());
+    }
+
+    #[test]
+    fn flat_iter() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 43);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys = [1, 4, 8];
+
+        for (key, value) in m.flat_iter() {
+            assert!(keys.contains(key));
+
+            if key == &1 {
+                assert!(value == &42 || value == &43);
+            } else {
+                assert_eq!(value, &42);
+            }
+        }
+    }
+
+    #[test]
+    fn flat_iter_mut() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 43);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys = [1, 4, 8];
+
+        for (key, value) in m.flat_iter_mut() {
+            assert!(keys.contains(key));
+
+            if key == &1 {
+                assert!(value == &42 || value == &43);
+
+                *value = 55;
+                assert_eq!(value, &55);
+            } else {
+                assert_eq!(value, &42);
+
+                *value = 76;
+                assert_eq!(value, &76);
+            }
+        }
+    }
+
+    #[test]
+    fn intoiterator_for_reference_type() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 43);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys = [1, 4, 8];
+
+        for (key, value) in &m {
+            assert!(keys.contains(key));
+
+            if key == &1 {
+                assert_eq!(value, &vec![42, 43]);
+            } else {
+                assert_eq!(value, &vec![42]);
+            }
+        }
+    }
+
+    #[test]
+    fn intoiterator_for_mutable_reference_type() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 43);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys = [1, 4, 8];
+
+        for (key, value) in &mut m {
+            assert!(keys.contains(key));
+
+            if key == &1 {
+                assert_eq!(value, &vec![42, 43]);
+                value.push(666);
+            } else {
+                assert_eq!(value, &vec![42]);
+            }
+        }
+
+        assert_eq!(m.get_vec(&1), Some(&vec![42, 43, 666]));
+    }
+
+    #[test]
+    fn intoiterator_consuming() {
+        let mut m: MultiMap<usize, usize> = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 43);
+        m.insert(4, 42);
+        m.insert(8, 42);
+
+        let keys = [1, 4, 8];
+
+        for (key, value) in m {
+            assert!(keys.contains(&key));
+
+            if key == 1 {
+                assert_eq!(value, vec![42, 43]);
+            } else {
+                assert_eq!(value, vec![42]);
+            }
+        }
+    }
+
+    #[test]
+    fn test_fmt_debug() {
+        let mut map = MultiMap::new();
+        let empty: MultiMap<i32, i32> = MultiMap::new();
+
+        map.insert(1, 2);
+        map.insert(1, 5);
+        map.insert(1, -1);
+        map.insert(3, 4);
+
+        let map_str = format!("{:?}", map);
+
+        assert!(map_str == "{1: [2, 5, -1], 3: [4]}" || map_str == "{3: [4], 1: [2, 5, -1]}");
+        assert_eq!(format!("{:?}", empty), "{}");
+    }
+
+    #[test]
+    fn test_eq() {
+        let mut m1 = MultiMap::new();
+        m1.insert(1, 2);
+        m1.insert(2, 3);
+        m1.insert(3, 4);
+        let mut m2 = MultiMap::new();
+        m2.insert(1, 2);
+        m2.insert(2, 3);
+        assert_ne!(m1, m2);
+        m2.insert(3, 4);
+        assert_eq!(m1, m2);
+        m2.insert(3, 4);
+        assert_ne!(m1, m2);
+        m1.insert(3, 4);
+        assert_eq!(m1, m2);
+    }
+
+    #[test]
+    fn test_eq_empty_key() {
+        let mut m1 = MultiMap::new();
+        m1.insert(1, 2);
+        m1.insert(2, 3);
+        let mut m2 = MultiMap::new();
+        m2.insert(1, 2);
+        m2.insert_many(2, []);
+        assert_ne!(m1, m2);
+        m2.insert_many(2, [3]);
+        assert_eq!(m1, m2);
+    }
+
+    #[test]
+    fn test_default() {
+        let _: MultiMap<u8, u8> = Default::default();
+    }
+
+    #[test]
+    fn test_from_iterator() {
+        let vals: Vec<(&str, i64)> = vec![("foo", 123), ("bar", 456), ("foo", 789)];
+        let multimap: MultiMap<&str, i64> = MultiMap::from_iter(vals);
+
+        let foo_vals: &Vec<i64> = multimap.get_vec("foo").unwrap();
+        assert!(foo_vals.contains(&123));
+        assert!(foo_vals.contains(&789));
+
+        let bar_vals: &Vec<i64> = multimap.get_vec("bar").unwrap();
+        assert!(bar_vals.contains(&456));
+    }
+
+    #[test]
+    fn test_from_vec_iterator() {
+        let vals: Vec<(&str, Vec<i64>)> = vec![
+            ("foo", vec![123, 456]),
+            ("bar", vec![234]),
+            ("foobar", vec![567, 678, 789]),
+            ("bar", vec![12, 23, 34]),
+        ];
+
+        let multimap: MultiMap<&str, i64> = MultiMap::from_iter(vals);
+
+        let foo_vals: &Vec<i64> = multimap.get_vec("foo").unwrap();
+        assert!(foo_vals.contains(&123));
+        assert!(foo_vals.contains(&456));
+
+        let bar_vals: &Vec<i64> = multimap.get_vec("bar").unwrap();
+        assert!(bar_vals.contains(&234));
+        assert!(bar_vals.contains(&12));
+        assert!(bar_vals.contains(&23));
+        assert!(bar_vals.contains(&34));
+
+        let foobar_vals: &Vec<i64> = multimap.get_vec("foobar").unwrap();
+        assert!(foobar_vals.contains(&567));
+        assert!(foobar_vals.contains(&678));
+        assert!(foobar_vals.contains(&789));
+    }
+
+    #[test]
+    fn test_extend_consuming_hashmap() {
+        let mut a = MultiMap::new();
+        a.insert(1, 42);
+
+        let mut b = HashMap::new();
+        b.insert(1, 43);
+        b.insert(2, 666);
+
+        a.extend(b);
+
+        assert_eq!(a.len(), 2);
+        assert_eq!(a.get_vec(&1), Some(&vec![42, 43]));
+    }
+
+    #[test]
+    fn test_extend_ref_hashmap() {
+        let mut a = MultiMap::new();
+        a.insert(1, 42);
+
+        let mut b = HashMap::new();
+        b.insert(1, 43);
+        b.insert(2, 666);
+
+        a.extend(&b);
+
+        assert_eq!(a.len(), 2);
+        assert_eq!(a.get_vec(&1), Some(&vec![42, 43]));
+        assert_eq!(b.len(), 2);
+        assert_eq!(b[&1], 43);
+    }
+
+    #[test]
+    fn test_extend_consuming_multimap() {
+        let mut a = MultiMap::new();
+        a.insert(1, 42);
+
+        let mut b = MultiMap::new();
+        b.insert(1, 43);
+        b.insert(1, 44);
+        b.insert(2, 666);
+
+        a.extend(b);
+
+        assert_eq!(a.len(), 2);
+        assert_eq!(a.get_vec(&1), Some(&vec![42, 43, 44]));
+    }
+
+    #[test]
+    fn test_extend_ref_multimap() {
+        let mut a = MultiMap::new();
+        a.insert(1, 42);
+
+        let mut b = MultiMap::new();
+        b.insert(1, 43);
+        b.insert(1, 44);
+        b.insert(2, 666);
+
+        a.extend(&b);
+
+        assert_eq!(a.len(), 2);
+        assert_eq!(a.get_vec(&1), Some(&vec![42, 43, 44]));
+        assert_eq!(b.len(), 2);
+        assert_eq!(b.get_vec(&1), Some(&vec![43, 44]));
+    }
+
+    #[test]
+    fn test_entry() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+
+        {
+            let v = m.entry(1).or_insert(43);
+            assert_eq!(v, &42);
+            *v = 44;
+        }
+        assert_eq!(m.entry(2).or_insert(666), &666);
+
+        assert_eq!(m[&1], 44);
+        assert_eq!(m[&2], 666);
+    }
+
+    #[test]
+    fn test_entry_vec() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+
+        {
+            let v = m.entry(1).or_insert_vec(vec![43]);
+            assert_eq!(v, &vec![42]);
+            *v.first_mut().unwrap() = 44;
+        }
+        assert_eq!(m.entry(2).or_insert_vec(vec![666]), &vec![666]);
+
+        assert_eq!(m[&1], 44);
+        assert_eq!(m[&2], 666);
+    }
+
+    #[test]
+    fn test_is_vec() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 1337);
+        m.insert(2, 2332);
+
+        assert!(m.is_vec(&1));
+        assert!(!m.is_vec(&2));
+        assert!(!m.is_vec(&3));
+    }
+
+    #[test]
+    fn test_macro() {
+        let mut manual_map = MultiMap::new();
+        manual_map.insert("key1", 42);
+        assert_eq!(manual_map, multimap!("key1" => 42));
+
+        manual_map.insert("key1", 1337);
+        manual_map.insert("key2", 2332);
+        let macro_map = multimap! {
+            "key1" =>    42,
+            "key1" =>  1337,
+            "key2" =>  2332
+        };
+        assert_eq!(manual_map, macro_map);
+    }
+
+    #[test]
+    fn retain_removes_element() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 99);
+        m.retain(|&k, &v| k == 1 && v == 42);
+        assert_eq!(1, m.len());
+        assert_eq!(Some(&42), m.get(&1));
+    }
+
+    #[test]
+    fn retain_also_removes_empty_vector() {
+        let mut m = MultiMap::new();
+        m.insert(1, 42);
+        m.insert(1, 99);
+        m.insert(2, 42);
+        m.retain(|&k, &v| k == 1 && v == 42);
+        assert_eq!(1, m.len());
+        assert_eq!(Some(&42), m.get(&1));
+    }
+}