// object provides fading ability and levels
Fadable {
	var track, level;
	fade {
		// exponential fading level toward <target> during <time> seconds
		// TODO: what if another fade routine is ongoing? => mischief
		|target, time|
		r {
			var rate = 1 / time * 0.05;
			while({((level - target).abs >= 0.01 )}, {
				if(
					(level >= target),
					{
						// fade out
						level = level - (rate.exp - 1);
						if((level < target), { level = target; });
					},
					{
						// fade in
						level = level + (rate.exp - 1);
						if((level > target), { level = target; });
					});
				0.05.wait;
				track.set(\amp, level);
			});
		}.play;
	}

	level_ {
		|amp|
		level = amp;
		track.set(\amp, level);
		^this
		// ("Amplitude in set to " << amp << "sec").postln;
	}
}