title: remove unwanted symbols in tagged_sents
body: <p>I'm using <code>nltk</code> <code>brown</code> corpus to get simplified tag set</p>
  
  <pre><code>import nltk
  from nltk.corpus import brown
  
  brown_tags = []
  
  for sent in brown.tagged_sents(categories = 'news', simplify_tags = True):
      brown_tags.extend([tag for (word, tag) in sent])
  
  tag_set = set(brown_tags)
  </code></pre>
  
  <p>then I get</p>
  
  <blockquote>
    <p>set(['', 'FW', 'DET', 'WH', "''", 'VBZ', 'VB+PPO', "'", ')', 'ADJ', 'PRO', '*', ',', '.', 'TO', 'NUM', 'NP', ':', 'ADV', '``', 'VD', 'VG', 'VN', 'N', 'P', 'EX', 'V', 'CNJ', 'UH', '(', 'MOD'])</p>
  </blockquote>
  
  <p>why there are <code>'', '""', ')',...</code>?
  how can I remove those symbols?</p>