Package Envelope{

	Multi-Stage(gate points)
	{
		time = z-1('0 time + speed) & (gate > 0)

		frac = time - Truncate(time)

		(speed y0) = Select(points Coerce(Int time))
		(next-speed y1) = Select(points Coerce(Int time) + 1i)
		Multi-Stage = y0 + frac * (y1 - y0)
	}
}