First phase of processing and analysis.

The "frontend" is the first phase of any analysis or transformation and consists of parsing the input language to produce an abstract syntax tree (AST). The nodes of the AST have types whose names begin with the letters "Sg" and are also known as "Sage nodes". The C++ type definitions are generated by a software layer named ROSETTA (found in $ROSE/src/ROSETTA), and specifically by a tool named CxxGrammarMetaProgram.