proposition: Lemma mxundefP T (x : mtx T) : mxundef_spec x (mxundef x) x.
proof: Proof. by case: x=>[||[t|]]; constructor. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq) pcm(options prelude pred) pcm(pcm morphism natmap)