@slack1 in-parallel allows one line in a for*/fold to efficiently iterate over more than one sequence at once: (for*/fold ([best 0] [seen-at 0] #:result (list best seen-at)) ([(t index) (in-parallel (in-list tuples) (in-naturals))] [now (in-value (log-identity (first t) (second t)))] #:when (< best now)) (values now index))

if you’re just wanting to include natural-number indices, in-indexed can be useful as well (i.e. instead of in-parallel and in-naturals, which is equivalent): (for*/fold ([best 0] [seen-at 0] #:result (list best seen-at)) ([(t index) (in-indexed (in-list tuples))] [now (in-value (log-identity (first t) (second t)))] #:when (< best now)) (values now index))

also minor FYI in case you were unaware, using in-list (or whatever appropriate in- form to iterate over a sequence) will be a lot more efficient than relying on dynamic dispatch to figure out what kind of sequence it is (in your example tuples was relying on dynamic dispatch)