reserve c,c1,c2 for Cardinal, G,G1,G2 for _Graph, v for Vertex of G;

theorem
  for E being set, G2 being reverseEdgeDirections of G1, E
  holds G1 is regular iff G2 is regular
proof
  let E be set, G2 be reverseEdgeDirections of G1, E;
  consider F being PGraphMapping of G1, G2 such that
    A1: F = id G1 & F is isomorphism by GLIBPRE0:77;
  thus thesis by A1, Th40;
end;
