intros a b; case a; case b; simpl in |- *; auto.
intros a1 m1 a2 m2 H1 H2; split; auto.
apply (eqA_trans _ _ _ _ _ _ _ _ _ cs) with (y := plusA A0 a1); auto.
apply (eqA_trans _ _ _ _ _ _ _ _ _ cs) with (y := plusA a1 A0); auto.
apply (eqA_sym _ _ _ _ _ _ _ _ _ cs); auto.
Qed.

(* Specifications: *)
Theorem plusTerm_comp_l :
forall a b c : Term,
eqT a c -> eqT b c -> eqTerm a b -> eqTerm (plusTerm a c) (plusTerm b c).