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_3(X /\ Y) c= proj1_3 X /\ proj1_3 Y
proof
  proj1_3(X /\ Y) c= proj1_3 X & proj1_3(X /\ Y) c= proj1_3 Y
       by Th10,XBOOLE_1:17;
 hence thesis by XBOOLE_1:19;
end;
