  A first order theory T is said to be "tight" if for any two deductively
closed extensions U and V of T (both of which are formulated in the language of
T), U and V are bi-interpretable iff U = V. By a theorem of Visser, PA (Peano
Arithmetic) is tight. Here we show that Z_2 (second order arithmetic), ZF
(Zermelo-Fraenkel set theory), and KM (Kelley-Morse theory of classes) are also
tight theories.
