The|xou filter [1] is used to check the existence of only one single element out of 2 possibles.
Examples:
[(#CHAPO|xou{#PS}) Either there is a stand-first, or a post-scriptum, but never both]
[(#CHAPO|xou{#PS}|non) There can be neither stand-first nor post-scriptum, or there can be both at the same time]
See also the |et and |ou filters.
See also
Other logical filters
test filters