theorem Th177:
  for F being PGraphMapping of G1, G2
  st F is isomorphism holds G1 is c-tcolorable iff G2 is c-tcolorable
proof
  let F be PGraphMapping of G1, G2;
  assume A1: F is isomorphism;
  then reconsider F0 = F as one-to-one PGraphMapping of G1, G2;
  F0" is isomorphism by A1, GLIB_010:75;
  hence G1 is c-tcolorable implies G2 is c-tcolorable by Th176;
  thus G2 is c-tcolorable implies G1 is c-tcolorable by A1, Th176;
end;
