Although set theory is the most popular foundation for mathematics, not
many mechanized mathematics systems are based on set theory. Zermelo-Fraenkel
(zf) set theory and other traditional set theories are not an adequate foundation for
mechanized mathematics. stmm is a version of von-Neumann-Bernays-G¨odel (nbg)
set theory that is intended to be a Set Theory for Mechanized Mathematics. stmm
allows terms to denote proper classes and to be undefined, has a definite description
operator, provides a sort system for classifying terms by value, and includes lambdanotation
with term constructors for function application and function abstraction.
This paper describes stmm and discusses why it is a good foundation for mechanized
mathematics.