sub lazy_merge3 (@list is copy) returns Sub {
    my $last = 0;

    my &by_n = -> $n { my $k = 0; -> $x { $x ?? $k += $n :: $k } };
    @list .= map:{ by_n $_ };

    sub {
        my $low;
        for @list -> $sub {
            my $val = $sub();
            $val = $sub(<next>) if $val <= $last;
            $low = $val unless $low && $low < $val;
        }
        $last = $low;
    }
}