docs: def trees_by_subpath(self, sub_path):
          """
          Search trees by `sub_path` using ``Tree.path.startswith(sub_path)``
          comparison.
  
          Args:
              sub_path (str): Part of the :attr:`.Tree.path` property of
                  :class:`.Tree`.
  
          Returns:
              set: Set of matching :class:`Tree` instances.
          """
          matches = (
              self.path_db[tree_path].keys()
              for tree_path in self.path_db.iterkeys()
              if tree_path.startswith(sub_path)
          )
  
          return set(sum(matches, []))