Re: Meaning of S-shaped bag delimiters

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.