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Re: [Lager] AD review of draft-ietf-lager-specification-10

Martin J. Dürst <duerst@it.aoyama.ac.jp> Mon, 14 March 2016 06:35 UTC

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From: "Martin J. Dürst" <duerst@it.aoyama.ac.jp>
To: Asmus Freytag <asmusf@ix.netcom.com>, Barry Leiba <barryleiba@computer.org>, draft-ietf-lager-specification@ietf.org
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Subject: Re: [Lager] AD review of draft-ietf-lager-specification-10
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Hello Barry, others,

On 2016/03/14 12:09, Asmus Freytag wrote:

> On 3/13/2016 7:57 AM, Barry Leiba wrote:
>> -- Section 6.2.5 --
>> Am I the only one who doesn't understand the distinction between
>> "difference" and "symmetric difference"?  I had assumed that
>> "difference" contained all those in one or the other, but not both...
>> but that seems to be what "symmetric difference" is (you say it's
>> xor).  Please explain (to me and in the document).

A difference is by its basic meaning never symmetric, because if A is 
*bigger* than B, then B is *smaller* than A, and so on.

> I have no opinion on whether you are the only one,

Me neither. I teach this stuff to undergrads, so I'd definitely have to 
be counted as 'biased'.

> but the term
> "symmetric-difference" defines what you think of as "difference", while
> the latter term means simply "subtract all the elements of one of the
> sets from their union" (normally you'd expect to see "from the other
> set", but that leaves unspecified how to subtract something that's not
> in a set to begin with).

Another way to explain it is that the difference between set A and set B 
is defined as all the elements in A that are not in B.
Formally: A \ B = { x | x∈A ∧ x∉B }
That's a direct definition which avoids to construct the union first.


> As these terms can be looked up in Wikipedia, with beautiful Venn
> diagrams, I'm not sure that we should spend time on defining them further.

Yes indeed, please see https://en.wikipedia.org/wiki/Set_(mathematics).

> However, perhaps a simple "(subtraction)" after "difference" will put
> the reader on the right track?

I suggest "set difference", or "set-theoretic difference". The operation 
is called "subtraction", the result "difference".

Regards,   Martin.

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