Creates an abstract syntax tree.
A successful call to this method returns all settings to their default values so the object is ready to be reused.
@param monitor the progress monitor used to report progress and request cancellation,or
null
if none
@return an AST node whose type depends on the kind of parserequested, with a fallback to a
CompilationUnit
in the case of severe parsing errors
@exception IllegalStateException if the settings providedare insufficient, contradictory, or otherwise unsupported