code: private void increment( TldFernFeature f , boolean positive ) {
  		if( positive ) {
  			f.incrementP();
  			if( f.numP > maxP )
  				maxP = f.numP;
  		} else {
  			f.incrementN();
  			if( f.numN > maxN )
  				maxN = f.numN;
  		}
  	}
docs: Increments the P and N value for a fern.  Also updates the maxP and maxN statistics so that it
  knows when to re-normalize data structures.