I agree that the problem is weird and stupid. It does highlight a lack of dynamic grouping[1] in lisp iteration tools though; as TFA suggests, arbitrary groupi

I agree that the problem is weird and stupid. It does highlight a lack of dynamic grouping[1] in lisp iteration tools though; as TFA suggests, arbitrary grouping is done idiomatically in lisp using an explicit associative type (usually a hash-table). Even my contribution (using the SERIES library) keeps the hash-table explicit. I don't know how much of this is habit, how much is lisp programmers being control freaks, and how much of it is "If I need to do grouping, having the hash-table explicitly available will probably be needed at some point anyways" 1: Static grouping (i.e. where all group keys are known in advance) is trivial.