fact: Record mixin_of (U : pcm) := Mixin { undef_op : U; unitb_op : U -> bool; _ : forall x, reflect (x = Unit) (unitb_op x); _ : ~~ valid undef_op; _ : forall x, undef_op \+ x = undef_op}.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)