mb event system 3 Sections EventSystems Doc
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Def mtag(m) == 1of(2of(m))

is mentioned by

Def w-tagged(tg; mss) == filter(ms.mtag(ms) = tg;mss)[w-tagged]
Def mtag(m) == mtag(m)[es-mtag]

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mb event system 3 Sections EventSystems Doc