theorem
   proj2 X \+\ proj2 Y c= proj2(X \+\ Y)
proof
  proj2 X \ proj2 Y c= proj2(X \ Y) & proj2 Y \ proj2 X c= proj2(Y \ X)
      by Th29;
  then proj2 X \+\ proj2 Y c= proj2(X\Y) \/ proj2(Y\X) by XBOOLE_1:13;
  hence thesis by Th27;
end;
