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 V. Kanovei and M. Reeken , Internal approach to external sets and universes, Part 1, Studia Logica, vol. 55 (1995), no. 2, pp. 229–257.
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 W. A. J. Luxemburg , What is nonstandard analysis?, American Mathematics Monthly, vol. 80 (Supplement) (1973), pp. 38–67.
 R. M. Solovay , A model of set theory in which every set of reals is Lebesgue measurable, Annals of Mathematics, vol. 92 (1970), pp. 1–56.