From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <aaebe89eb51daf9e4816b636ef595317@hamnavoe.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] ls, rc question
From: Richard Miller <rm@hamnavoe.com>
In-Reply-To: <34feff09e0eb478d71030dd09fcd8135@terzarima.net>
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Date: Fri, 19 Mar 2004 09:24:51 +0000
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>>>The thing is, reads from a union directory do in fact give you the union. 
> 
> i think the argument is provoked by the assumption that it's a union of sets, preventing duplicates,
> but the system happens to work with bags, not sets, where union can (must) allow duplicates.

Sorry, David is correct on this point.  Sets and bags are both
unordered, and union is therefore commutative.  But directories are
ordered -- the ordering is essential to resolve the ambiguity of
duplicate names.  So the accurate mathematical model is a sequence
(aka list), and bind is not union of sets or bags, but append of
sequences -- a non-commutative operator, hence the need to specify
MBEFORE or MAFTER.

-- Richard