anchor.diff:      instance/1,
       insert/3,
       fold/3,
  -    foldwide/3
  +    foldwide/3,
  +    walk/2,
  +    at/3,
  +    locate/2,
  +    locate/3
   ]).
   
   %%
  
positive.code: 
  %%
  
  newnode() ->
      {node, erstar_bound:empty(), []}.
  
  
negative.code: newleaf(Bound, Data) ->
      {Bound, Data}.
  
  nodetype({_, _, _}) ->
      node;
  
  nodetype({_, _}) ->
      leaf.
  
  bound({Bound, _}) ->
      Bound;
  
  