The inputs aren’t identical, though, once you get to 1 2.
[:+/\| is window-dressing for sum=: +/\,  while sumr =: [:+/\|. sums the 
reverse of its input. 
So,
   (sum ; sumr) 1 2
+---+---+
|1 3|2 3|
+---+---+

Does that solve the mystery?

Mike


Please reply to [email protected].      
Sent from my iPad

> On 4 Dec 2018, at 09:59, Linda Alvord <[email protected]> wrote:
> 
>    (([:+/\|)^:(i.6)1 1);([:+/\|.)^:(i.6)1 1
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