Lemma freeVarSubTerms3 : forall (n : nat) (ts : fol.Terms L n) (v : nat) (s : Term) (x : nat), In x (freeVarTerms L n (substituteTerms L n ts v s)) -> In x (list_remove _ eq_nat_dec v (freeVarTerms L n ts)) \/ In x (freeVarTerm L s).