Resid -- A theory of residuals

Ole Rasmussen has ported to Isabelle/ZF the Coq proofs described in the article

@Article{huet-residual,
  author  = "{G\'erard} Huet",
  title   = "Residual Theory in $\lambda$-Calculus: A Formal Development",
  journal = JFP,
  year    = 1994,
  volume  = 4,
  number  = 3,
  pages   = "371--394"}
See Rasmussen's report The Church-Rosser Theorem in Isabelle: A Proof Porting Experiment.