Hai Dan, thanks for your reply. Now in general, let me see if I understand this right: an expression is always dynamic, 'active' so to speak. It will never stop generating values. So it's impossible for an expression which, let's say, generates a layer rotation over time with a constant speed, to output one certain value at a certain point in the timeline as a 'fixed' value. Like a snapshot function: measured data of one single moment in time to be put in some kind of passive container after which the active link with the expression is disconnected, so to speak.
So what I tried to describe above , is not possible using expressions?
Thanks for taking the trouble to answer, this question has everything to do with the function sampleImage, which I only want to use on the first frame of a timeline generating an alpha value. This value should be set on frame zero and after that remain unchanged.