fact: Fixpoint unzip z (e : env) : env := match z with | [::] => e | (x, inl e') :: z' => unzip z' (ENode x e' e) | (x, inr e') :: z' => unzip z' (ENode x e e') end.
imports: HB(structures) BinPos BinNat mathcomp(ssreflect ssrbool ssrfun ssrnat eqtype seq bigop)