theorem
  proj2_3 X \+\ proj2_3 Y c= proj2_3(X \+\ Y)
proof
  proj2_3 X \ proj2_3 Y c= proj2_3(X \ Y) &
  proj2_3 Y \ proj2_3 X c= proj2_3(Y \ X) by Th37;
  then proj2_3 X \+\ proj2_3 Y c= proj2_3(X\Y) \/ proj2_3(Y\X) by XBOOLE_1:13;
 hence thesis by Th35;
end;
