Does the compiler build the initial AST based on a grammar, before the transformations happen? That would mean you can't expand the grammar with user-defined rules. The reason you can "implement for loops in user land" is because they're already part of the grammar, is that correct?
How abstract is that initial AST? Can you use these rewriting transformations to expand the grammar?
How abstract is that initial AST? Can you use these rewriting transformations to expand the grammar?