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 Th15:
 [x,y,z] in X implies y in proj2_3 X
 proof
  assume [x,y,z] in X;
   then [x,y] in proj1 X by Def12;
  hence thesis by Def13;
 end;
