theorem
  for X, Y being complex-membered set holds
    addRel(X,z) \/ addRel(Y,z) c= addRel(X \/ Y,z)
proof
  let X, Y be complex-membered set;
  X c= X \/ Y & Y c= X \/ Y by XBOOLE_1:7;
  then addRel(X,z) c= addRel(X \/ Y,z) & addRel(Y,z) c= addRel(X \/ Y,z)
    by Th13;
  hence thesis by XBOOLE_1:8;
end;
