code: public Counter<L> logProbabilityOf(Datum<L, F> example) {
      if(example instanceof RVFDatum<?, ?>)return logProbabilityOfRVFDatum((RVFDatum<L,F>)example);
      Counter<L> scores = scoresOf(example);
      Counters.logNormalizeInPlace(scores);
      return scores;
    }
docs: Returns a counter mapping from each class name to the log probability of
  that class for a certain example.
  Looking at the the sum of e^v for each count v, should be 1.0.