code: function() {
        var N = this.data.length;
        var num_train = Math.floor(this.train_ratio * N);
        this.folds = []; // flush folds, if any
        for(var i=0;i<this.num_folds;i++) {
          var p = randperm(N);
          this.folds.push({train_ix: p.slice(0, num_train), test_ix: p.slice(num_train, N)});
        }
      }
docs: sets this.folds to a sampling of this.num_folds folds