Ok, now I got a issue here. When developing the gui filter - I am not exactly sure how do I get the number of layer inputs to affect the dynamic options. It seems to treat $! as one every time.
u "{$1}_"{$!>=1?2:0}\
"{$2}_"{$!>=1?2:0}\
"{$3}"\
"{$4}"\
"{$5}"\
"{$6}"\
"{$7}"\
"{$8}"