> If I were to do this in Scheme, I'd use a "parameter" called 'current-chip' (think of it like a dynamically scoped variable) and write getter/setter procedures that operate on the current chip.

Do you mean to create global functions with those generic, unqualified names? The unhygienic solution at least keeps them nicely inside the lexical scope of the "with" construct. That actually looks more hygienic, for another, more traditional, meaning of hygienic.