Easiest way to change what to prepend?

I have a list which will change over time, and I want to prepend a number (using a number box) in the front and output this as a new list. The new output only changes when the original list changes, that is, if I only change the number I want to prepend, the output remains without sending out a bang. What is the easiest way to do so?

For append, this can be achieved by using [zl join], but what I want is something like reversing the two inlets of [zl join] so that only the right inlet triggers an output.