anchor.diff:      two_tuple(ListType, ListType);
   lists_mod_return_type(usort, 1, Vst) ->
       same_length_type({x,0}, Vst);
  -lists_mod_return_type(usort, 2, Vst) ->
  -    same_length_type({x,1}, Vst);
   lists_mod_return_type(zip, 2, _Vst) ->
       list;
  -lists_mod_return_type(zip3, 3, _Vst) ->
  -    list;
   lists_mod_return_type(zipwith, 3, _Vst) ->
  -    list;
  -lists_mod_return_type(zipwith3, 4, _Vst) ->
       list;
   lists_mod_return_type(_, _, _) ->
       term.
  
positive.code: lists_mod_return_type(reverse, 1, Vst) ->
      same_length_type({x,0}, Vst);
  lists_mod_return_type(seq, 2, _Vst) ->
      list;
  lists_mod_return_type(seq, 3, _Vst) ->
      list;
  lists_mod_return_type(sort, 1, Vst) ->
      same_length_type({x,0}, Vst);
  
negative.code:     list;
  lists_mod_return_type(flatten, 1, _Vst) ->
      list;
  lists_mod_return_type(map, 2, Vst) ->
      same_length_type({x,1}, Vst);
  lists_mod_return_type(MF, 3, Vst) when MF =:= mapfoldl; MF =:= mapfoldr ->
  