I interpret this as using the binary operation "both" in a reduction over a list of three items. Kind of like how you can add a list of numbers through e.g. (PL

I interpret this as using the binary operation "both" in a reduction over a list of three items. Kind of like how you can add a list of numbers through e.g. (PLUS 1 2 3 4) in LISP, although addition is actually a binary operation.