anchor.diff:      fold/3,
       foldwide/3,
       walk/2,
  +    leaves/1,
  +    size/1,
       at/3,
       around/4,
       locate/2,
  
positive.code: 
  walk(WalkFun, {?MODULE, _, Root}) ->
      walk_node(WalkFun, [], Root).
  
  %%
  
  
negative.code:     DY = CY - RY,
      (DX * DX + DY * DY) =< Dist2.
  
  always_true(_, _) ->
      true.
  
  %%
  
  maybe_reroot(Roots) when is_list(Roots) ->
  