Without them, we get this huge list
of SRFIs for different sequence-like types, and thus potentially a
R7RS-large with a huge number of procedures. That way lies madness.
Yeah, I guess I thought the spirit of SRFI 121 generators was that they would be the lingua franca of R7RS-large sequences. So each subsequent sequence data type would simply provide a ->generator procedure, and client code would call that then use any of the many varied iteration procedures in SRFI 121. Specifying each of the filter, fold, etc. variations for every data structure seems IMO overly heavyweight.
Kevin Wortman