A List-machine Benchmark for Mechanized Metatheory
          by Andrew W. Appel and Xavier Leroy

This is a Twelf solution to the benchmark, with proofs removed
  as an exercise for the reader.  Start with sources.cfg.

See also:

http://www.inria.fr/rrrt/rr-5914.html
http://www.cs.princeton.edu/~appel/listmachine/
http://gallium.inria.fr/~xleroy/listmachine/