Thanks for noticing!
I didn't know about and while I was doing this exercise and found it only later in solutions of others. And after some time I feel it is confusing even for me to understand my "all id"

Oh, right, thanks a lot.

use let in the definition of next so that the sum is only computed once (the compiler is likely to optimize, but still).

You can use and instead of all id, since both are based on folding (&&)