code: public AnnotatedSentence addAnnotation(String annotationName, Object value) {
      Map<String, Object> newAnnotations = Maps.newHashMap(annotations);
      newAnnotations.put(annotationName, value);
      return new AnnotatedSentence(words, posTags, newAnnotations);
    }
docs: Gets a copy of this annotated sentence containing the
  provided annotation in addition to the annotations on
  {@code this}.
  
  This method overwrites any other annotation with the same
  name.
  
  @param annotationName
  @param value
  @return