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[1] Here's what the mksh manual says,

    For historical reasons, open and close braces may be used instead
    of in and esac, for example: “case $foo { (ba[rz]|blah) date ;; }”

and OpenBSD's ksh manual says,

    For historical reasons, open and close braces may be used instead of
    in and esac e.g. case $foo { *) echo bar; }.