code: def default_filter_filename(filename):
      # type: (Optional[str]) -> Optional[str]
      
      if filename is None:
          return None
      elif filename.startswith(TOP_DIR):
          if filename.startswith(TOP_DIR_DOT):
              # Skip subdirectories starting with dot (e.g. .vagrant).
              return None
          else:
              # Strip current directory and following slashes.
              return filename[TOP_DIR_LEN:].lstrip(os.sep)
      elif filename.startswith(os.sep):
          # Skip absolute paths not under current directory.
          return None
      else:
          return filename
docs: Default filter for filenames.
  
      Returns either a normalized filename or None.
      You can pass your own filter to init_types_collection().