optimization for sum metaop on a Range

This seems to work with my limited testing locally.

TimToady said:

<blockquote>
11:01 &lt; TimToady&gt; lee_: there's something a little wrong with it in regards to flattening, since a range in list context 
                  flattens, but you're kinda bypassing the flattening, such that [+] $range would "flatten" $range when it 
                  shouldn't, seems to me
</blockquote>

But I'm not sure I understand.