theorem
  for G2 being G1-isomorphic _Graph
  holds G1.vChromaticNum() = G2.vChromaticNum()
proof
  let G2 be G1-isomorphic _Graph;
  consider F being PGraphMapping of G1, G2 such that
    A1: F is isomorphism by GLIB_010:def 23;
  thus thesis by A1, Th75;
end;
