Section gram4.
Variable X V1 R1 : Ensf.
Variable S1 : Elt.
Variable V2 R2 : Ensf.
Variable S2 : Elt.
Variable S : Elt.
Let C := Gunion_disj_p V1 R1 S1 V2 R2 S2 S.
Let Vu := fst C.
Let C' := snd C.
Let Ru := fst C'.
Let Su := snd C'.

Lemma inter_X_V1_u_V2 :
isGram X V1 R1 S1 -> isGram X V2 R2 S2 -> inter X (union V1 V2) empty.