AST Generators are intended to make a abstract syntax of a given file. This is usually to make the parsing later a lot easier and clearer.
The AST generator for PDDL files generally makes ASTNodes from sets of "(" and ")". Additionally it also saves the line context of where the nodes start at.
This AST Generator generates ASTNodes from the node structure in the Fast Downward Translator Format. Basically it matches nodes between "?" marks.