anchor.diff:  
   
   %% @doc
  -%%
  +%% Return an iterator that tranverses the pieces in the scarcity
  +%% list in the same order as the list returned by :to_list/1.
   %% @end
   -spec iterator(#scarcity{}) -> term().
   iterator(Scarcity) ->
  
positive.code: 
  
  %% @doc
  %%
  %% @end
  -spec decrement(pos_integer(), #scarcity{}) -> scarcity().
  decrement(Index, Scarcity) ->
  
negative.code: 
  
  %% @doc
  %% Ensure that a piece index is not negative, it really shouldn't be.
  %% @end
  -spec assert_positive(pos_integer()) -> ok.
  assert_positive(Integer) ->
  