1. Boffa, M.: The point on Quine's NF (with a bibliography). TeoriaIV (2), 3?13 (1984)
2. Bowen, K.A.: Model theory for modal logic. Kripke models for modal predicate calculi, vol. 127 of Synthese Library. Dordrecht, Boston, Lancaster: Reidel 1979
3. Crabbé, M.: Ambiguity and stratification. Fundam. Math.101, 11?17 (1978)
4. Crabbé, M.: On NFU. Notre Dame J. Formal Logic (to appear)
5. Fraenkel, A.A., Bar-Hillel, Y., Levy, A.: Foundation of set theory. Amsterdam, London: North-Holland 1973