docs: function entropy(labels, ix) {
      var N= ix.length;
      var p=0.0;
      for(var i=0;i<N;i++) {
          if(labels[ix[i]]==1) p+=1;
      }
      p=(1+p)/(N+2); // let's be bayesian about this
      q=(1+N-p)/(N+2);
      return (-p*Math.log(p) -q*Math.log(q));
  }