I'd like to do some research on the AST in clang. Where do I find the root
of the AST? It seems that void ParseAST(Sema &S, bool PrintStats = false);
generates the AST, but where is it saved?

I'd like to do some modification on the AST. E.g. exchange all
BinaryOperator "+" with "-" before the a.out is generated.

Kind regards