func funcDeriv(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
	mat := vals[0].(Matrix)

	for _, samples := range mat {
		// No sense in trying to compute a derivative without at least two points.
		// Drop this Vector element.
		if len(samples.Points) < 2 {
			continue
		}

		// We pass in an arbitrary timestamp that is near the values in use
		// to avoid floating point accuracy issues, see
		// https://github.com/prometheus/prometheus/issues/2674
		slope, _ := linearRegression(samples.Points, samples.Points[0].T)
		enh.out = append(enh.out, Sample{
			Point: Point{V: slope},
		})
	}
	return enh.out
}
func funcDeriv(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
	mat := vals[0].(Matrix)

	for _, samples := range mat {
		// No sense in trying to compute a derivative without at least two points.
		// Drop this Vector element.
		if len(samples.Points) < 2 {
			continue
		}

		// We pass in an arbitrary timestamp that is near the values in use
		// to avoid floating point accuracy issues, see
		// https://github.com/prometheus/prometheus/issues/2674
		slope, _ := linearRegression(samples.Points, samples.Points[0].T)
		enh.out = append(enh.out, Sample{
			Point: Point{V: slope},
		})
	}
	return enh.out
}
// === deriv(node ValueTypeMatrix) Vector ===