proposition: Lemma inl_prop5 S i x: inc i (isI S) -> inc x (Vg (isE S) i) -> Vf (Vg (isf S) (J i i)) x = x.
proof: Proof. move => iI xE; rewrite (is_identity_f iI) idV //. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)