proposition: Lemma Is_zero: forall t, is_zero t = true -> t = zero.
proof: Proof. intros t H. destruct t; auto; discriminate. Qed.
imports: ATBR(Common) ATBR(Classes) ATBR(Graph) ATBR(SemiLattice) ATBR(SemiRing) ATBR(KleeneAlgebra) ATBR(MxGraph) ATBR(Reification)