Kripke–Platek set theory with urelements

Kripke–Platek set theory (KP) is a foundational system in set theory that serves as a framework for discussing sets and their properties. It is particularly notable for its treatment of sets without the full power of the axioms found in Zermelo-Fraenkel set theory (ZF). KP focuses on sets that can be constructed and defined in a relatively restricted manner, making it suitable for certain areas of mathematical logic and philosophy.