sub merge {
        my ($f, $s, @r) = @_;
        push @r => shift @{$$f [0] < $$s [0] ? $f : $s} while @$f && @$s;
       (@r, @$f, @$s);
    }
    sub merge_sort;
    sub merge_sort {
        @_ <= 1 ? @_ : merge [merge_sort @_ [0 .. @_ / 2 - 1]],
                             [merge_sort @_ [@_ / 2 .. $#_]]
    }