theorem Th63:
  for G being loopless _Graph, H being Subgraph of G
  holds H.vChromaticNum() c= G.vChromaticNum()
proof
  let G be loopless _Graph, H be Subgraph of G;
  G is G.vChromaticNum()-vcolorable by Th54;
  hence thesis by Th57;
end;
