anchor.diff:  
   -opaque t()   ::  state_orset:state_orset().
   
  --export_type[(t/0)].
  +-export_type([t/0]).
   
   %% API
   -export([add/3]).
  
positive.code: %% @doc
  %% @end
  %% -----------------------------------------------------------------------------
  to_list(T) ->
      sets:to_list(state_orset:query(T)).
  
  
negative.code: %% @doc
  %% @end
  %% -----------------------------------------------------------------------------
  -spec new() -> t().
  
  new() ->
      state_orset:new().
  
  