theorem
  Ex(x,y,z,p) = Ex(t,s,q) implies x = t & y = s & Ex(z,p) = q
proof
  assume
A1: Ex(x,y,z,p) = Ex(t,s,q);
  hence x = t by Th13;
  Ex(y,z,p) = Ex(s,q) by A1,Th13;
  hence thesis by Th13;
end;
