theorem
  proj2_4 X \+\ proj2_4 Y c= proj2_4(X \+\ Y)
proof
  proj2_4 X \ proj2_4 Y c= proj2_4(X \ Y) &
  proj2_4 Y \ proj2_4 X c= proj2_4(Y \ X) by Th45;
  then proj2_4 X \+\ proj2_4 Y c= proj2_4(X\Y) \/ proj2_4(Y\X) by XBOOLE_1:13;
 hence thesis by Th43;
end;
