method eval_exp : exp -> ('r, 'a) m
method eval_var : var -> ('r, 'a) m
method eval_int : word -> ('r, 'a) m
method eval_unop : unop -> exp -> ('r, 'a) m
method eval_cast : cast -> int -> exp -> ('r, 'a) m
method eval_ite : cond:exp -> yes:exp -> no:exp -> ('r, 'a) m
method eval_concat : exp -> exp -> ('r, 'a) m
method eval_unknown : string -> typ -> ('r, 'a) m