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