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

theorem Th24:
  RED(a,1) = a & RED(1,a) = 1
proof
  a,1 are_coprime by Th2;
  then a hcf 1 = 1 by Th20;
  hence thesis by ORDINAL3:71;
end;
