docs: def nested_update(d, u):
      """Merge two nested dicts.
  
      Nested dicts are sometimes used for representing various recursive structures. When
      updating such a structure, it may be convenient to present the updated data as a
      corresponding recursive structure. This function will then apply the update.
  
      Args:
        d: dict
          dict that will be updated in-place. May or may not contain nested dicts.
  
        u: dict
          dict with contents that will be merged into ``d``. May or may not contain
          nested dicts.
  
      """
      for k, v in list(u.items()):
          if isinstance(v, collections.Mapping):
              r = nested_update(d.get(k, {}), v)
              d[k] = r
          else:
              d[k] = u[k]
      return d