Effect.ADSRADSR (Attack/Decay/Sustain/Release) envelopes.
val make : int -> (float * float * float * float) -> tCreate an envelope with specified Attack/Decay/Sustain/Release times in seconds (excepting sustain which is an amplification coefficient between 0. and 1.). Negative sustain means that that notes should be released just after decay.
val init : unit -> stateInitial state for processing.
val dead : state -> bool