Additional evidence gathered: For a more contemporary use, a quick search of arXiv for |\rbag| turned up
https://arxiv.org/html/2501.11689v3#S3.E4 If ζ is a data sequence, we let ⟅ζ⟆ stand for the bag of its elements. ------------------------------------------------------------------------ ... Corneli, the sole evidence cited for encoding, does not define ⟅⟆ but seems to be using those to parenthesize propositions. It does seem that more recent papers use it for multisets, see also https://arxiv.org/html/2405.16035v1#S4 To simplify notations, we also use ⟅ ⟆ to denote a multiset by enumerating each element as many times as its multiplicity. For example, A=⟅a, a, b⟆ contains a with multiplicity 2 and b with multiplicity 1. For brevity, we identify a multiset (A, m) with the set A if m ≡ 1, e.g. ⟅a, b, c⟆={a,b,c}. ------------------------------------------------------------------------ ...the notation is older eg stmaryrd fonts (using pre-unicode custom encoding) document this in the first paragraph https://tex.org.uk/fonts/stmaryrd/stmaryrd.pdf Image <https://private-user-images.githubusercontent.com/1268738/533638140-e8e643a8-17d2-46b3-bddf-32673788145a.png?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.Nqs46Tu2EUON6WG3_QKXoi_HBT5fkjRtXN0UHaKFy9I> That version of the doc is from 2004, but I don't think the font has changed (other than the format changing from metafont to type1) since the first release in 1994.
