| Select of rvar * set_exp * boole_exp
| Union of set_exp * set_exp
| Intersect of set_exp * set_exp
| Diff of set_exp * set_exp
| Select of rvar * set_exp * boole_exp
| Union of set_exp * set_exp
| Intersect of set_exp * set_exp
| Diff of set_exp * set_exp