proposition: Lemma model_of_cnf c s : s \in val c -> is_cnf (drop_sign s).
proof: Proof. Proof using cnf_S. move => A. exact: cnf_S (valP c). Qed.
imports: Relations Classical_Pred_Type mathcomp.ssreflect.ssreflect mathcomp(all_ssreflect) CompDecModal.libs(edone bcase fset base) CompDecModal.CPDL(PDL_def)