proposition: Lemma mulNmor (phi : 'Mor(M, N)) (psi : 'Mor(N, P)) : (- phi) ** psi = - (phi ** psi).
proof: Proof. by apply: val_inj => /=; rewrite mulNmx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq path) mathcomp(ssralg fintype perm choice matrix bigop zmodp mxalgebra poly) ssrcomplements stronglydiscrete coherent edr