groupby.p6

sub groupby(@elements is copy, $key={ $_ }) is export {
    gather {
        my @rest = @elements.Array;
        while ?@rest {
            my $head = $key(shift @rest);
            @rest = (@rest ==> grep { $head ne $key($_)});
            take (@rest ==> grep { $head eq $key($_)}, $head);
        }
    }
}

say groupby(['a','b','a','a','a','b','b','c','a']); #((a) (b) (c))
Should be ((a a a a a) (b b b) (c))