Lemma sound_boxp_stable {L: Language} {minL: MinimunLanguage L} {mL: ModalLanguage L} {MD: Model} {kMD: KripkeModel MD} {M: Kmodel} {R1: KM.Relation (Kworlds M)} {R2: SS.Relation (Kworlds M)} {R1_bis: Bisimulation SS.Krelation KM.Krelation} {SM: Semantics L MD} {kmSM: KripkeModalSemantics L MD M SM} {stableSM: SemanticStable L MD M SM}: forall x: expr, semantic_stable x -> semantic_stable (boxp x).