fact: Definition join' x y := match x, y with undef, _ => undef | _, undef => undef | nown, x => x | x, nown => x | mx _, mx _ => undef end.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq) pcm(options prelude pred) pcm(pcm morphism natmap)