def _concat_tuples(tagging):
    """
  param: tuples of [(word, tag), ...]
  return: string, 'w1/t1 w2/t2 ...'
  """
    TOKEN_BLANK = ' '
    wl = []
    for x, y in tagging:
        wl.append(x + '/' + y)
    concat_str = TOKEN_BLANK.join(wl)
    return concat_str
