reserve x,x1,x2,x3,x4,y,y1,y2,y3,y4,z,z1,z2,z2,z4 for object;
reserve X,X1,X2,X3,X4,Y for set;

theorem
  proj1 X \+\ proj1 Y c= proj1(X \+\ Y)
proof
  proj1 X \ proj1 Y c= proj1(X \ Y) & proj1 Y \ proj1 X c= proj1(Y \ X)
     by Th25;
  then proj1 X \+\ proj1 Y c= proj1(X\Y) \/ proj1(Y\X) by XBOOLE_1:13;
 hence thesis by Th23;
end;
