Changes by: Xin Yu (xiny at cs.caltech.edu)
Date: 2006-03-25 14:47:59 -0800 (Sat, 25 Mar 2006)
Revision: 8953
Log message:

      Made some progres on the nth_prefix lemma. Now it remains to prove bind_subst_nth_prefix_wf_aux0; unfortunately, I couldn't figure out how. bind_substl_nth_prefix_nth_suffix and bind_substl_nth_prefix can be ignored.