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+# Common Expression Language
+
+The Common Expression Language (CEL) implements common semantics for expression
+evaluation, enabling different applications to more easily interoperate.
+
+Key Applications
+
+*   Security policy: organizations have complex infrastructure and need common
+    tooling to reason about the system as a whole
+*   Protocols: expressions are a useful data type and require interoperability
+    across programming languages and platforms.
+
+
+Guiding philosophy:
+
+1.  Keep it small & fast.
+    *   CEL evaluates in linear time, is mutation free, and not Turing-complete.
+        This limitation is a feature of the language design, which allows the
+        implementation to evaluate orders of magnitude faster than equivalently
+        sandboxed JavaScript.
+2.  Make it extensible.
+    *   CEL is designed to be embedded in applications, and allows for
+        extensibility via its context which allows for functions and data to be
+        provided by the software that embeds it.
+3.  Developer-friendly.
+    *   The language is approachable to developers. The initial spec was based
+        on the experience of developing Firebase Rules and usability testing
+        many prior iterations.
+    *   The library itself and accompanying toolings should be easy to adopt by
+        teams that seek to integrate CEL into their platforms.
+
+The required components of a system that supports CEL are:
+
+*   The textual representation of an expression as written by a developer. It is
+    of similar syntax to expressions in C/C++/Java/JavaScript
+*   A representation of the program's abstract syntax tree (AST).
+*   A compiler library that converts the textual representation to the binary
+    representation. This can be done ahead of time (in the control plane) or
+    just before evaluation (in the data plane).
+*   A context containing one or more typed variables, often protobuf messages.
+    Most use-cases will use `attribute_context.proto`
+*   An evaluator library that takes the binary format in the context and
+    produces a result, usually a Boolean.
+
+For use cases which require persistence or cross-process communcation, it is
+highly recommended to serialize the type-checked expression as a protocol
+buffer. The CEL team will maintains canonical protocol buffers for ASTs and
+will keep these versions identical and wire-compatible in perpetuity:
+
+*  [CEL canonical](https://github.com/google/cel-spec/tree/master/proto/cel/expr)
+*  [CEL v1alpha1](https://github.com/googleapis/googleapis/tree/master/google/api/expr/v1alpha1)
+
+
+Example of boolean conditions and object construction:
+
+``` c
+// Condition
+account.balance >= transaction.withdrawal
+    || (account.overdraftProtection
+    && account.overdraftLimit >= transaction.withdrawal  - account.balance)
+
+// Object construction
+common.GeoPoint{ latitude: 10.0, longitude: -5.5 }
+```
+
+For more detail, see:
+
+*   [Introduction](doc/intro.md)
+*   [Language Definition](doc/langdef.md)
+
+Released under the [Apache License](LICENSE).