Parallel Transform AST

Question

I am writing a simple ast parser, and I have already created a transformer in visitor pattern, and I have a question, how do I transform AST in parallel, my parser is a recursive decent parser, so it is hard to become parallel, but I think transformer may be easier to become parallel, since visitor already has entry and exit method for any kind of ast type. Is there are concepts or algorithms I can learn for building a parallel transformer?