docs: def _exists(filenames):
      """Check if every filename exists. If not, print an error
      message and remove the item from the list.
  
      Parameters
      ----------
      filenames : list
          List of filenames to check for existence.
  
      Returns
      -------
      list
          Filtered list of filenames that exists.
      """
      exists = []
      for filename in filenames:
          if os.path.isfile(filename):
              exists.append(filename)
          else:
              print('fijibin ERROR missing output file {}'.format(filename))
  
      return exists