prolog [ isGram2 union_inter ] 5.
(*Intro G1. Intro G2. Apply union_inter. Apply isGram2 with R1 S1; Assumption. Apply isGram2 with R2 S2; Assumption.*)
Qed.

Lemma inter_X_Vu_d :
isGram X V1 R1 S1 -> isGram X V2 R2 S2 -> ~ dans S X -> inter X Vu empty.