Transforming parse tree to AST

Once we can parse a program's syntax, we convert it into a semantic representation called an "abstract syntax tree" (AST).

Parrot Compiler Toolkit provides a set of generic AST nodes:

• PAST::Node - base class
• PAST::Op - basic operations
• PAST::Val - values (typically constants)
• PAST::Var - variables
• PAST::Stmts - collection of statements
• PAST::Block - lexical scopes and closures

Each node may have attributes and other PAST nodes as children.