code: def remove(self, item):
          
          if item in self:
              super(NGram, self).remove(item)
              del self.length[item]
              for ngram in self.splititem(item):
                  del self._grams[ngram][item]
docs: Remove an item from the set. Inverts the add operation.
  
          >>> from ngram import NGram
          >>> n = NGram(['spam', 'eggs'])
          >>> n.remove('spam')
          >>> list(n)
          ['eggs']