作者：Philip Potter

规范

P40 (**) Goldbach's conjecture.
    Goldbach's conjecture says that every positive even number greater
    than 2 is the sum of two prime numbers. Example: 28 = 5 + 23. It is
    one of the most famous facts in number theory that has not been proved
    to be correct in the general case. It has been numerically confirmed
    up to very large numbers (much larger than we can go with our Prolog
    system). Write a predicate to find the two prime numbers that sum up
    to a given even integer.

示例

> say ~goldbach 28
5 23

源代码： P40-rhebus.pl

use v6;

subset Even of Int where * %% 2;

sub goldbach (Even $n where * > 2 ) {
    for 2..$n/2 -> $k {
        if $k.is-prime && ($n-$k).is-prime {
            return ($k, $n-$k);
        }
    }
    return; # fail
}

say ~ goldbach $_ for 28, 36, 52, 110, 62710560;