and are documented in its specification. These are details that require a context to determine their validity and behaviour. For example, if a language allows new types to be declared, a CFG cannot predict the names of such types nor the way in which they should be used. Even if a language has a predefined set of types, enforcing proper usage usually requires some context. Another example is duck typing, where the type of an element can change depending on context. Operator overloading is yet another case where correct usage and final function are context-dependent. Design The design of an AST is often closely linked with the design of a compiler and its expected features. Core requirements include the following: Variable types must be preserved, as well as the location of each declaration in source code. The order of executable statements must be explicitly represented and well defined. Left and right components of binary operations must be stored and correctly identified. Identifiers and their assigned values must be stored for assignment statements. These requirements can be used to design the data structure for the AST. Some operations will always require two elements, such as the two terms for addition. However, some language constructs require an arbitrarily large number of children, such as argument lists passed to programs from the command shell. As a result, an AST used to represent code written in such a language has to also be flexible enough to allow for quick addition of an unknown quantity of children. To support compiler verification it should be possible to unparse an AST into source code form. The source code produced should be sufficiently similar to the original in appearance and identical in execution, upon recompilation. The AST is used intensively during semantic analysis, where the compiler checks for correct usage of the elements of the program and the language. The compiler also generates symbol tables based on the AST during semantic analysis. A complete traversal of the tree allows verification of the correctness of the program. After verifying correctness, the AST serves as the base for code generation. The AST is often used to generate an intermediate representation (IR), sometimes called an intermediate language, for the code generation. Other usages AST differencing AST differencing, or for short tree differencing, consists of computing the list of differences between two ASTs. This list of differences is typically called an edit