proposition: Lemma Exercise1_3a1: sgraph E13_sF.
proof: Proof. move=> t /setUb_P [y]; rewrite /fam_of_substrates ! Lgd => yd. by rewrite /disjointU_fam LgV; [ case /indexed_P | aw]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)