title: Python nltk stemmers never remove prefixes
body: <p>I'm trying to preprocess words to remove common prefixes like "un" and "re", however all of nltk's common stemmers seem to completely ignore prefixes:</p>
  
  <pre><code>from nltk.stem import PorterStemmer, SnowballStemmer, LancasterStemmer
  
  PorterStemmer().stem('unhappy')
  # u'unhappi'
  
  SnowballStemmer('english').stem('unhappy')
  # u'unhappi'
  
  LancasterStemmer().stem('unhappy')
  # 'unhappy'
  
  PorterStemmer().stem('reactivate')
  # u'reactiv'
  
  SnowballStemmer('english').stem('reactivate')
  # u'reactiv'
  
  LancasterStemmer().stem('reactivate')
  # 'react'
  </code></pre>
  
  <p>Isn't part of the job of a stemmer to remove common prefixes as well as suffixes? Is there another stemmer which does this reliably?</p>