reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;

theorem Th15:
  a hcf b = {} implies a = {}
proof
  a hcf b divides a by Def5;
  then a = (a hcf b)*^(a div^ (a hcf b)) by Th7;
  hence thesis by ORDINAL2:35;
end;
