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 Th16:
 X c= Y implies proj1_4 X c= proj1_4 Y
proof assume X c= Y;
  then proj1_3 X c= proj1_3 Y by Th10;
 hence thesis by Th8;
end;
