@Adám I don’t believe there is a way to “make a list” in K which leaves lists alone. You’d need a conditional. x~*x tests if you’re an atom. There was a discussion adjacent to this recently on Discord. Maybe (#x)#x?
For the upcoming Quest, why should F0 be (0 1)? Shouldn't it be nothing? 1 is not less than or equal to 0. Even (0 1) (1 1) seems to make more sense than (0 1).
I think by this definition fractions always have a numerator and a denominator, thus the reduced representation of 0 is 0/1. 1/1 is not not in range because you’re looking for numbers up to 0.