In his 1991 dissertation, Olin Shivers introduces a concept of control flow graphs for functional languages, provides an algorithm to statically derive a safe approximation of the control flow graph and proves this algorithm correct. In our student research project, Shivers' algorithms and proofs are formalized using the theorem prover system Isabelle. This package contains the Haskell prototype of the Isabelle formalization, together with some pretty printing and rendering facilities. It is provided as a reference, not as a ready-to-use library.