feature request: preserve ordering of input elements

[archenemies]

This behavior is somewhat confusing:

$ echo "<a>hello</a><span>bye</span>"| pup 'a,span'
<a>
 hello
</a>
<span>
 bye
</span>
$ echo "<a>hello</a><span>bye</span>"| pup 'span,a'
<span>
 bye
</span>
<a>
 hello
</a>

I expected Pup to do one pass over the input, and to interpret the "," operator like a logical OR. In other words, the output for the second command should be the same as the output for the first command.

Someone else commented on this recently.