* think about a better system for EXTVARs, separate \set and \get functions are clunky
* impl_def_val: allow num_exp
* fix right recursion of let_exp and set_exp
* think about a better system for EXTVARs, separate \set and \get functions are clunky
* impl_def_val: allow num_exp
* fix right recursion of let_exp and set_exp