The `fox` procedure would make a wonderful case study for keyword
arguments in SRFIs, if we can come to an agreement on how to do them.
The answer here is SRFI 159/166. :)
That said, if Scheme wasn't expressive enough for such a combinator language, keywords would have come handy for a procedure like `fox'. So this is also a wonderful case study for general keyword arguments vs some tailored DSL.