In his later years, Jack Schwartz devoted much energy to the implementation of a proof-checker based on set theory and to the preparation of a large script file to be fed into it. His goal was to attain a verified proof of the Cauchy integral theorem of complex analysis.
This contribution to the memorial volume for Jack reflects that effort: it briefly reports the chronicle of his proof-checking project and highlights some features of the system as implemented; in an annex, it presents a proof scenario leading from bare set theory to two basic theorems about claw-free graphs.
