 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
  for X being integer-membered set holds
    [i1,i2] in addRel(X,i) iff i1 in X & i2 in X & i2 = i + i1 by Th11;
