code: public void addMention(Mention mention)
      {
          if (!mention.getSentence().equals(this))
              throw new IllegalArgumentException();
          mentions.add(mention);
      }
docs: Adds a {@link Mention} to this Sentence, ignoring any potential overlap with existing {@link Mention}s. Normally called by instance of
  {@link Tagger} or post-processors.
  
  @param mention