docs: public boolean isConstantResponse() {
      double m = Double.NaN;
      for( int b=0; b<_bins.length; b++ ) {
        if( _bins[b] == 0 ) continue;
        if( var(b) > 1e-14 ) return false;
        double mean = mean(b);
        if( mean != m )
          if( Double.isNaN(m) ) m=mean;
          else if(Math.abs(m - mean) > 1e-6) return false;
      }
      return true;
    }