ISF animation logic help

Trying to port an old quartz composition to ISF, but I’m a bit rusty when it comes to textual programming. Very simple comp that just cyclicly animates the alpha of a png logo. Exponential fade-in, holds it for x_amount of seconds, exponential fade-out, then repeats after y_amount of minutes. I found the ‘Set Alpha.fs’ demo in ISFEditor, but am having trouble remembering how to do the animation aspect. I think a forLoop is in order here? I miss Quartz Composer waaaaaaaaaaaaah! :)