anchor.diff:  
   walk(WalkFun, {?MODULE, _, Root}) ->
       walk_node(WalkFun, [], Root).
  +
  +%%
  +
  +-spec leaves(tree()) -> [treeleaf()].
  +
  +leaves(RTree) ->
  +    walk(fun always_true/2, RTree).
  +
  +%%
  +
  +-spec size(tree()) -> non_neg_integer().
  +
  +size({?MODULE, _, Root}) ->
  +    count_leaves(0, Root).
   
   %%
   
  
positive.code:     fold/3,
      foldwide/3,
      walk/2,
      at/3,
      around/4,
      locate/2,
  
negative.code:     DY = CY - RY,
      (DX * DX + DY * DY) =< Dist2.
  
  always_true(_, _) ->
      true.
  
  %%
  
  maybe_reroot(Roots) when is_list(Roots) ->
  