anchor.diff:  %% Main entry point.
   -export([module/2]).
   
  --import(ordsets, [from_list/1,union/2]).
   -import(lists,   [member/2,foldl/3,foldr/3]).
   
   -type fa() :: {atom(), arity()}.
  
positive.code: 
  import({Mod,Fs}, St) ->
      true = is_atom(Mod),
      Mfs = from_list(Fs),
      St#expand{imports=add_imports(Mod, Mfs, St#expand.imports)}.
  
  add_imports(Mod, [F|Fs], Is) ->
  
negative.code:     PreExp=PreDef,
      {[gen_beh_info(Callbacks, OptionalCallbacks)],
       St#expand{defined=gb_sets:union(gb_sets:from_list(PreDef), Defined),
  	       exports=ordsets:union(ordsets:from_list(PreExp), Exports)}}.
  
  gen_beh_info(Callbacks, OptionalCallbacks) ->
      List = make_list(Callbacks),
  