The |ou filter [1] is used to check the existence of at least one of two possible elements.
Examples:
[(#CHAPO|ou{#PS}) There is either a stand-first or a post-scriptum (or both) ]
[(#CHAPO|ou{#PS}|non) There is neither a stand-first nor a post-scriptum, and clearly then not both of them either]
See also
other logical filters
test filters