fact: Definition edge (pg : PreGraph) (n n' : Vertex) : Prop := vvalid pg n /\ vvalid pg n' /\ step pg n n'.
imports: Coq.Logic.ProofIrrelevance CertiGraph.lib.Ensembles_ext CertiGraph.lib.EquivDec_ext CertiGraph.lib.List_ext CertiGraph.lib.relation_list CertiGraph.lib.Equivalence_ext