A Fixedpoint Approach to Implementing (co)inductive Definitions

The paper briefly describes a method of formalizing non-well- founded data structures in standard ZF set theory. Examples include lists of n elements, the accessible part of a relation and the set of primitive recursive functions. One example of a coinductive definition is bisimulations for lazy lists. Recursive datatypes are examined in detail, as well as one example of a codatatype: lazy lists. The appendices are simple user's manuals for this Isabelle/ZF package.