anchor.diff:  %% filter(Fun, OrdSet) -> OrdSet.
   %%  Filter OrdSet with Fun.
   
  --spec filter(fun((_) -> boolean()), ordset(_)) -> ordset(_).
  +-spec filter(fun((T) -> boolean()), ordset(T)) -> ordset(T).
   
   filter(F, Set) ->
       lists:filter(F, Set).
  
positive.code: %% del_element(Element, OrdSet) -> OrdSet.
  %%  Return OrdSet but with Element removed.
  
  -spec del_element(term(), ordset(_)) -> ordset(_).
  
  del_element(E, [H|Es]) when E > H -> [H|del_element(E, Es)];
  del_element(E, [H|_]=Set) when E < H -> Set;
  
negative.code: %% add_element(Element, OrdSet) -> OrdSet.
  %%  Return OrdSet with Element inserted in it.
  
  -spec add_element(E, ordset(T)) -> [T | E,...].
  
  add_element(E, [H|Es]) when E > H -> [H|add_element(E, Es)];
  add_element(E, [H|_]=Set) when E < H -> [E|Set];
  