It depends on exactly what you're doing, but it might be like this:
s = effect("Slider Control")("Slider");
if (time < key(2).time)
ease (time, key(1).time, key(2).time, [s,key(1).value[1]],key(2).value)
else if (time < key(3).time)
value
else
ease(time, key(3).time, key(4).time, key(3).value, [s,key(4).value[1]])
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