anchor.diff:  bif_return_type(is_binary, [_], _) -> bool;
   bif_return_type(is_float, [_], _) -> bool;
   bif_return_type(is_function, [_], _) -> bool;
  +bif_return_type(is_function, [_,_], _) -> bool;
   bif_return_type(is_integer, [_], _) -> bool;
   bif_return_type(is_list, [_], _) -> bool;
   bif_return_type(is_map, [_], _) -> bool;
  
positive.code: bif_return_type(is_tuple, [_], _) -> bool;
  %% Misc.
  bif_return_type(tuple_size, [_], _) -> {integer,[]};
  bif_return_type(node, [], _) -> {atom,[]};
  bif_return_type(node, [_], _) -> {atom,[]};
  bif_return_type(hd, [_], _) -> term;
  
negative.code: math_mod_return_type(pi, 0) -> {float,[]};
  math_mod_return_type(F, A) when is_atom(F), is_integer(A), A >= 0 -> term.
  
  lists_mod_return_type(all, 2, _Vst) ->
      bool;
  lists_mod_return_type(any, 2, _Vst) ->
      bool;
  lists_mod_return_type(keymember, 3, _Vst) ->
      bool;
  lists_mod_return_type(member, 2, _Vst) ->
      bool;
  lists_mod_return_type(prefix, 2, _Vst) ->
      bool;
  lists_mod_return_type(suffix, 2, _Vst) ->
      bool;
  lists_mod_return_type(dropwhile, 2, _Vst) ->
      list;
  lists_mod_return_type(duplicate, 2, _Vst) ->
      list;
  lists_mod_return_type(filter, 2, _Vst) ->
      list;
  lists_mod_return_type(flatten, 1, _Vst) ->
      list;
  lists_mod_return_type(flatten, 2, _Vst) ->
      list;
  