# How to draw the box of Dijkstra's Guarded Command Language?

I'm wondering whether there is a standard, canonical way to typeset programs in Dijkstra's GCL (EWD 472). In particular, I'm interested in a command producing a symbol for the box [] in

Ideally, the box should be also visible as some meaningful Unicode symbol (or symbols) in the text layer of the resulting PDF file, rather than the usual gibberish you get from NFSS symbols.

Currently, I simply draw the box with \fbox around appropriate white \rules, which is kinda extremely ad hoc. Detexify gives zero usable results, and shapecatcher gives way too many equally-looking white rectangular boxes, none of which has a description anywhere near Dijkstra's symbol.

Try 2AFF.
the symbol should be present in the stix fonts with the name \bigtalloblong. that's not a particularly good name either, and i've suggested to the maintainer of the unicode-math package that maybe something better can be devised as an alternative.