Parse an ast file using compiler instance

Hi ,
I'm writing a small program that transforms source into an AST file (for merging with other files later) . I am now trying to read the ast file , and parse the AST. I have been able to read the file into an ASTUnit object and am now trying to parse the AST using a compiler instance .

I have already written a front-end action, ast consumer and a recursive visitor and have attempted to parse the ast using the method from the Recursive Visitor tutorial but to no avail. Can anyone show me how I would parse an ast using the ASTUnit and Compiler Instance(If that is the right way)?