code: protected void bprop(float target) {
        assert (target != missing_real_value);
        if (params.loss != Loss.MeanSquare) throw new UnsupportedOperationException("Regression is only implemented for MeanSquare error.");
        final int row = 0;
        // Computing partial derivative: dE/dnet = dE/dy * dy/dnet = dE/dy * 1
        final float g = target - _a.get(row); //for MSE -dMSE/dy = target-y
        float m = momentum();
        float r = _minfo.adaDelta() ? 0 : rate(_minfo.get_processed_total()) * (1f - m);
        bprop(row, g, r, m);
      }
docs: Backpropagation for regression
  @param target floating-point target value