fact: Definition dualgraph: PreGraph V E := Build_PreGraph EV EE (vvalid g) (evalid g) dual_src dual_dst.
imports: Coq.Classes.Equivalence Coq.Classes.EquivDec VST.msl.Coqlib2 CertiGraph.graph.graph_model CertiGraph.graph.path_lemmas CertiGraph.graph.reachable_ind CertiGraph.graph.subgraph2