theorem Th33:
  for G2 being reverseEdgeDirections of G1, E
  holds G1 is c-vcolorable iff G2 is c-vcolorable
proof
  let G2 be reverseEdgeDirections of G1, E;
  thus G1 is c-vcolorable implies G2 is c-vcolorable by Lm2;
  G1 is reverseEdgeDirections of G2, E by GLIB_007:3;
  hence thesis by Lm2;
end;
