| Expr_mul of whenexpr * whenexpr (** arithmetic multiplication *)
| Expr_div of whenexpr * whenexpr (** arithmetic division *)
| Expr_mod of whenexpr * whenexpr (** arithmetic modulo *)
+ | Expr_len of whenexpr (** length *)
| Expr_changes of string (** changes var *)
| Expr_increases of string (** increases var *)
| Expr_decreases of string (** decreases var *)