[EM] Summability criterion: do I have this right?

[Rob Lanphier]

Hi folks,

I've made a change to electowiki's "Summability criterion" article:
https://electowiki.org/wiki/Summability_criterion

Here's the chunk that I added:

> For batch summability to be true, the following must be true:
>
>    - Say that candidates "A", "B", and "C" run against each other in an
>    arbitrary election using single-winner electoral system "S"
>    - Say that candidate "A" wins in batch "X" (or precinct "X") when
>    ballots are tabulated using single-winner electoral system "S"
>    - Say that candidate "A" wins in batch "Y" (or precinct "Y") when
>    ballots are tabulated using single-winner electoral system "S"
>    - Therefore, candidate "A" must win when batches "X" and "Y" of
>    ballots are tabulated using single-winner electoral system "S" for "S" to
>    be "batch summable" (and thus, pass the "summability criterion")
>
> (end of definition)

Am I correct?  I'm trying to come up with a definition that is easy enough
for a layperson to understand, but is also accurate.  I realize that this
definition only captures a subset of elections that demonstrate summability
problems, but this seems (to me) like the core of the problem with
summability.

Rob
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