func primes_with_descending_digits(base = 10) {

    var list = []
    var digits = @(1..^base)

    var end_digits = digits.grep { .is_coprime(base) }
    list << digits.grep { .is_prime && !.is_coprime(base) }...

    for k in (0 .. digits.end) {
        digits.combinations(k, {|*a|
            var v = a.digits2num(base)
            end_digits.each {|d|
                var n = (v*base + d)
                next if ((n >= base) && (a[0] <= d))
                list << n if n.is_prime
            }
        })
    }

    list.sort
}

var base = 10
var arr = primes_with_descending_digits(base)

say "There are #{arr.len} descending primes in base #{base}.\n"

arr.each_slice(8, {|*a|
    say a.map { '%9s' % _ }.join(' ')
})