This is not done over regular lisp lists, but over scheme syntax objects that retain the original source info. Those syntax objects are also the basis of the h

This is not done over regular lisp lists, but over scheme syntax objects that retain the original source info. Those syntax objects are also the basis of the hygienic macro systems in many schemes (at least ones using syntax-case) so that macros also benefit from that information. The lisp source representation is already an AST, so these kinds of transformations are trivial.