It would be really nice to have an AST that was a tree with ~. This would have countless advantages, like the ast-fold could consume the AST and only allocate if it is actually creating a new node, and we'd generally not have the problems associated with Gc (the GC might be able to land!).

However, there are a few areas that are very tricky; mainly the AST map, which maps node_id to actual AST nodes of various types, the only safe way to do this with an ~ tree is with & references... which freezes the AST for the remainder of the program, so no transformations like constant evaluation, and no consuming of the AST by trans.