docs: def get_accumulator_dir(cachedir):
      '''
      Return the directory that accumulator data is stored in, creating it if it
      doesn't exist.
      '''
      fn_ = os.path.join(cachedir, 'accumulator')
      if not os.path.isdir(fn_):
          # accumulator_dir is not present, create it
          os.makedirs(fn_)
      return fn_