Changes by: Aleksey Nogin (nogin at cs.caltech.edu)
Date: 2005-12-23 12:40:17 -0800 (Fri, 23 Dec 2005)
Revision: 8355
Log message:

      This merges Itt_list2!nth with Itt_list3!nth_elem. The new operation is called
      Itt_list2!nth, but is defined the way Itt_list3!nth_elem used to be defined ---
      namely nth{l;2} is unconditionally equal to hd{tl{tl{l}}.