docs: def is_subdir(child, parent):
      """
      Determine if "child" is a subdirectory of "parent".
  
      If child == parent, returns True.
      """
      child_path = os.path.realpath(child)
      parent_path = os.path.realpath(parent)
  
      if len(child_path) < len(parent_path):
          return False
  
      for i in range(len(parent_path)):
          if parent_path[i] != child_path[i]:
              return False
  
      return True