The |et
filter [1] (since SPIP 2.0) is used to check the existence of both of two elements.
Wikipedia offers a complete definition of the Logical conjunction.
Examples:
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[(#CHAPO|et{#PS}) There is a stand-first and a post-scriptum ]
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[(#CHAPO|et{#PS}|non) There is not both a stand-first and a post-scriptum (but there may be one or the other) ]