 reserve X, Y for set, A for Ordinal;
 reserve z,z1,z2 for Complex;
 reserve r,r1,r2 for Real;
 reserve q,q1,q2 for Rational;
 reserve i,i1,i2 for Integer;
 reserve n,n1,n2 for Nat;

theorem
  [n1,n1+n] in addRel(NAT,n)
proof
  n1 in NAT & n1+n in NAT by ORDINAL1:def 12;
  hence thesis by Th11;
end;
