anchor.diff:  %% union([OrdSet]) -> OrdSet
   %%  Return the union of the list of ordered sets.
   
  --spec union([ordset(_)]) -> ordset(_).
  +-spec union([ordset(T)]) -> ordset(T).
   
   union([S1,S2|Ss]) ->
       union1(union(S1, S2), Ss);
  
positive.code: %% filter(Fun, OrdSet) -> OrdSet.
  %%  Filter OrdSet with Fun.
  
  -spec filter(fun((_) -> boolean()), ordset(_)) -> ordset(_).
  
  filter(F, Set) ->
      lists:filter(F, Set).
  
negative.code: %% union(OrdSet1, OrdSet2) -> OrdSet
  %%  Return the union of OrdSet1 and OrdSet2.
  
  -spec union(ordset(T1), ordset(T2)) -> ordset(T1 | T2).
  
  union([E1|Es1], [E2|_]=Set2) when E1 < E2 ->
      [E1|union(Es1, Set2)];
  