theorem Th121:
  G is edgeless iff G.eChromaticNum() = 0
proof
  hereby
    assume G is edgeless;
    then G.size() = 0;
    hence G.eChromaticNum() = 0 by Th120, XBOOLE_1:3;
  end;
  assume G.eChromaticNum() = 0;
  then G is 0-ecolorable by Lm14;
  hence thesis;
end;
