Changes by: Aleksey Nogin (nogin at cs.caltech.edu)
Date: 2005-11-11 03:31:16 -0800 (Fri, 11 Nov 2005)
Revision: 8162
Log message:

      Added a rewrite lemma to reduce: length{tail{'l; 'n} ~ 'n
      This significantly simplifes the proofs of the tail induction lemmas.