theorem Th168:
  for G2 being reverseEdgeDirections of G1, E
  holds G1 is c-tcolorable iff G2 is c-tcolorable
proof
  let G2 be reverseEdgeDirections of G1, E;
  thus G1 is c-tcolorable implies G2 is c-tcolorable by Lm17;
  G1 is reverseEdgeDirections of G2, E by GLIB_007:3;
  hence thesis by Lm17;
end;
