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

theorem Th23:
  a,b are_coprime implies RED(a,b) = a
proof
  assume a,b are_coprime;
  then a hcf b = 1 by Th20;
  hence thesis by ORDINAL3:71;
end;
