anchor.diff:          {H,T1} -> split(Num-1,T1,[H|Acc])
       end.
   
  +-spec view(tree(Key,Val)) -> nil | {{Key,Val},tree(Key,Val)}.
   
   view({ByVal,ByKey,Comp}) ->
       case gb_trees:is_empty(ByVal) of
  
positive.code:              dict:erase(Key,ByKey),
               Comp}
      end.
  
  store(Key,Val,T = {ByVal,ByKey,Comp}) ->
      case dict:find(Key,ByKey) of
  
negative.code:     end.
  
  add_key_to_val_idx(Key,Val,ByVal) ->
      Keys = gb_trees:get(Val,ByVal),
      Keys1 = gb_sets:add(Key,Keys),
      gb_trees:update(Val,Keys1, ByVal).
  