Actuarial mathematics makes frequent use of a notation that as far as I know isn’t used anywhere else, and that is a bracket enclosing the northeast corner of a symbol.
This is used as subscript of another symbol, such as an a or an s, and there may be other decorations such more superscripts or subscripts.
For typesetting in LaTeX, the bracket is the only part that’s not standard. If you can put a bracket around a symbol, you can make the bracketed symbol a subscript just as you’d make anything else a subscript. LaTeX package actuarialangle lets you do this.
The angle notation that actuaries use for annuity-related calculations is not used anywhere else that I know of, but a variation was once used for factorials back in the 1800s, putting a bracket around the southwest corner of a symbol rather than the northeast. You can see examples here, including one use by David Hilbert.
Related post: Floor, ceiling, bracket