fact: Definition pos := fix loop (e : env) p {struct e} := match e, p with | ENode t _ _, 1%positive => t | ENode t e _, (p~0)%positive => loop e p | ENode t _ e, (p~1)%positive => loop e p | _, _ => idx end.
imports: HB(structures) BinPos BinNat mathcomp(ssreflect ssrbool ssrfun ssrnat eqtype seq bigop)