fact: Definition union h1 h2 := if (h1, h2) is (Def hs1 ns1, Def hs2 ns2) then if disj hs1 hs2 then Def (@un_nullP _ _ ns1 ns2) else Undef else Undef.
imports: Coq(ssreflect ssrbool ssrfun Eqdep) mathcomp(ssrnat eqtype seq path) pcm(options axioms finmap) pcm(pcm unionmap)