code: @Override
    public double plus(double w1, double w2) {
      if (!isMember(w1) || !isMember(w2)) {
        return Double.NEGATIVE_INFINITY;
      }
  
      return w1 + w2;
    }
docs: /*
  (non-Javadoc)
  
  @see
  edu.cmu.sphinx.fst.weight.AbstractSemiring#Plus(edu.cmu.sphinx.fst.weight
  .float, edu.cmu.sphinx.fst.weight.float)