code: func (as AnnotatedSentence) Tags() []POSTag {
  	retVal := make([]POSTag, len(as))
  	for i, a := range as {
  		retVal[i] = a.POSTag
  	}
  	return retVal
  }
docs: // Tags returns the POSTags of the sentence. The return value has exactly the same length as the sentence.