intros a b; case a; case b; simpl in |- *; auto.
Qed.

Theorem plusTerm_assoc :
forall a a0 A1 : Term,
eqT A1 a0 ->
eqT a a0 ->
eqTerm (plusTerm (plusTerm A1 a0) a) (plusTerm A1 (plusTerm a0 a)).