The first sentence only refer to one comparator, while the second sentence uses plural "comparators" and I was confused. My guess is that the second sentence implies both a comparator returned by (srfi 146)'s make-mapping-comparator *and* a comparator returned by (srfi 146 hash)'s make-mapping-comparator, but can't it be "The comparator is obliged to provide ..."?   (If English-fluent readers don't think it confusing, I'm ok with it.)