In Quantification in Non-Classical Logic, Gabbay et al. use the curly wedge symbol ($\curlywedge$) to denote a void/empty sequence (see page 20 in the Google Books preview; it's introduced on page xviii in the introduction).

Is this an established/common notation? Does anyone know where the notation comes from, assuming it isn't novel to this book?

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    $\begingroup$ I haven't heard of that. Sometimes I've seen a capital lambda ($\Lambda$) for an empty sequence, but that's the closest I've got. $\endgroup$ – Henning Makholm Jan 6 '18 at 20:43
  • $\begingroup$ @HenningMakholm Thanks for the input, I wouldn't be surprised if this usage is derivative on the use of capital lambda -- with curly wedge being used as an italicized variant or something of the sort. (It would make sense that they wouldn't want to use $\Lambda$ since they use it as a metavariable for an arbitrary logic.) $\endgroup$ – Dennis Jan 6 '18 at 20:50

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