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 Th21:
 [x1,x,x2,x3] in X implies x in proj2_4 X
proof
 assume [x1,x,x2,x3] in X;
  then [[x1,x],x2,x3] in X;
  then [x1,x] in proj1_3 X by Th13;
 hence thesis by Def13;
end;
