let s_index = function
| Formula.S_Type -> 0
| Formula.S_Cons -> 1
| Formula.S_Logic_Sig -> 2
| Formula.S_Logic_Def -> 3
| Formula.S_Logic_Prop -> 4
| Formula.S_Model_Sig -> 5
| Formula.S_Model_Def -> 6
| Formula.S_Model_Prop -> 7
| Formula.S_User_Sig -> 8
| Formula.S_User_Prop -> 9