

Four edges are unreachable.

  In del_ele, edges 3 and 5 correspond to a check for
  the case where the function is called with a null list
  or null element.  This can't happen in the current program,
  where the function is only called after checks for
  nonempty list/element.  The code is there as good
  programming practice, to catch cases where programmer
  has a call to this where a null list/element could come in.

  The same goes for find_nth, in which edges 4 and 6 are unreachable.

Note, however, that in modified versions, the edges may
become reachable, if code that catches the cases checked
for is removed or modified.
