reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;

theorem
  Ex(x,y,z,p) = Ex(t,s,q) implies x = t & y = s & Ex(z,p) = q
proof
  Ex(x,y,z,p) = Ex(x,y,Ex(z,p));
  hence thesis by Th17;
end;
