tree construction from text string

it would be great to have a tree construction thing that lets me build an AST from "a=3;" raw text using a grammar for the target language. Also you can hook in ST to it so you can build a tree from "<lhs>=<rhs>". This only seems useful during tree parsing as you can't have the parser invoke itself to build an AST; the recursive definition wouldn't work in general. During tree rewriting, however, you could use this nicely.