 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
  addRel(INT,i1) * addRel(INT,i2) = addRel(INT,i1+i2)
proof
  A1: addRel(INT,i1) * addRel(INT,i2) c= addRel(INT,i1+i2) by Th17;
  now
    let x,y be object;
    reconsider a=x,b=y as set by TARSKI:1;
    assume A2: [x,y] in addRel(INT,i1+i2);
    then [a,b] in addRel(INT,i1+i2);
    then A3: a in INT & b in INT by MMLQUER2:4;
    then reconsider a,b as Integer;
    A4: b = i1 + i2 + a by A2, Th11;
    set c = i1 + a;
    c in INT & b = i2 + c by A4, INT_1:def 2;
    then [a,c] in addRel(INT,i1) & [c,b] in addRel(INT,i2) by A3, Th11;
    hence [x,y] in addRel(INT,i1) * addRel(INT,i2) by RELAT_1:def 8;
  end;
  then addRel(INT,i1+i2) c= addRel(INT,i1) * addRel(INT,i2) by RELAT_1:def 3;
  hence thesis by A1, XBOOLE_0:def 10;
end;
