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

theorem Th16:
  a hcf a = a & a lcm a = a
proof
  reconsider c = a as Element of omega by ORDINAL1:def 12;
  for b st b divides a & b divides a holds b divides c;
  hence a hcf a = a by Def5;
  for b st a divides b & a divides b holds c divides b;
  hence thesis by Def4;
end;
