The theory of hereditarily finite sets is formalised, following
An HF set is a finite collection of other HF sets; they enjoy an induction principle
and satisfy all the axioms of ZF set theory apart from the axiom of infinity, which is negated.
All constructions that are possible in ZF set theory (Cartesian products, disjoint sums, natural numbers,
functions) without using infinite sets are possible here.
The definition of addition for the HF sets follows Kirby.
This development forms the foundation for the Isabelle proof of Gödel's incompleteness theorems,
which has been formalised separately
[2015-02-23] Added the theory "Finitary" defining the class of types that can be embedded in hf, including int, char, option, list, etc.