fact: Definition merge_union_bwdEr x (p : x \in U :|: V) (inV : x \in V) : x \notin U -> merge_union_bwd (Sub x p) = inr (Sub x inV).
imports: HB(structures) Coq(Setoid CMorphisms) mathcomp(all_ssreflect) GraphTheory(preliminaries bij)