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Subject: Re: [xdi] Proposal for separation of subject/predicate andparent-subject/child-subject semantics

Hello Markus and Drummond,

thanks for your replies. I think there is however some confusion which  
I want to clarify once for all. I'm not saying that +a/+b is  
resolution equivalent to +a+b. I think we are now all agreeing on this  
(at least I think :-).
Logical equivalence is a different concept. It means that if +a/+b is  
resolvable then also +a+b must be. Note that they DO NOT resolve to  
the SAME node (this is instead resolution equivalence).
The requirement I'm talking about (if +a/+b is resolvable then also  
+a+b must be and viceversa) is needed to ensure consistency in my  
semantic model. For instance, having in a graph


will allow to deal with =example+email as the set of emails which  
=example owns (=example+email), and consequently make assertions on  
them (pls refer to  
http://lists.oasis-open.org/archives/xdi/201004/msg00004.html for  
details on this, btw I can clarify them in next phc). This is also  
highlighted in point 1) of  

Note that resolution of certain statements is needed, even if not  
asserted in the graph, but inferred using a reasoner (again pls refer  
to my mail for details) in order to implement what we have discussed in


>> =bill.barnhill: We can get the graph containing statements about  
>> any statement in the graph, and that graph will only exist if and  
>> only if the statement is in the graph. put more simply (+a/+b) is  
>> resolvable, or R(+a/+b) = True. Generalizing this we see that if  
>> R(X) = True then R((X)) = True, meaning  and (X) are logically  
>> equivalent
>> Drummond: It seems the import of what you are saying is that +a <=> (+a)

>> giovanni: resolution = your statement is true; fault = your  
>> statement is false

>> =bill.barnhill: but not  +a = (+a)
>> Drummond: Yes!
>> Drummond: A $is statement in an XDI graph establishes what Bill  
>> calls "resolution equivalence"

where we used symbol <=> to address logical equivalence, and = to  
address resolution equivalence.

Now my point is: either the TC has a different semantic model to  
propose for XDI (in this case, I would be glad to know more and  
discuss about it), or the TC wants to discard semantic consistency for  
XDI. If the latter is the case, then, frankly, I think the TC is  
underestimate a tremendously powerful feature which could instead  
change the whole concept of web applications in next few years. Please  
let me know your opinion about it.

Thank you,

Def. Quota "Markus Sabadello" <markus.sabadello@xdi.org>:

> What I'm afraid of is that people will never understand resolution
> equivalence, and that it would be difficult to implement at the storage and
> messaging levels of an XDI stack.
> Can't we just have logical equivalence without resolution equivalence?
> On the logical level a high level application/reasoner/etc can infer lots of
> stuff based on some $is, $has, $is$a, etc rules we make up, but I'm really a
> bit scared of having to implement such extra XDI "magic" just for plain XDI
> storage and messaging..
> Markus
> On Tue, Apr 20, 2010 at 5:21 PM, Giovanni Bartolomeo <
> giovanni.bartolomeo@uniroma2.it> wrote:
>> Dear Drummond, All,
>> thank you for this proposal - two main thoughts from my side:
>> 1) it is FUNDAMENTAL that if +a/+b is resolvable then also +a+b MUST be,
>> and viceversa. We used the symbol <==> to say this, +a/+b <==> +a+b.
>> Obviously the stored document can only contain one of the two, e.g. the
>> document only contains +a/+b, but +a+b MUST be resolvable as well.
>> This is about the relationship between asserted and inferred statements
>> which I illustrated in the second part of my email
>> http://lists.oasis-open.org/archives/xdi/201004/msg00004.html - we didn't
>> discuss this yet because we ran out of time last week, but it would be good
>> to address this in next phc.
>> 2) About the following statements (in which the symbol <==> is used in a
>> somehow different way than in point 1)
>>  +a/$has/+b      <==>    +a/+b
>>> +a/+/+b <==>    +a+b
>>> +a/$/$b <==>    +a$b
>>> +a/=/=b <==>    +a=b
>>> +a/@/@b <==>    +a@b
>>> +a/*/*b <==>    +a*b
>>> +a/!/!b <==>    +a!b
>>> +a/()/(b)       <==>    +a(b)
>> note that the statements on the left side in their own give origin to
>> metastatements like +a/$has/+, +a/$has/$, +a/$has/() which in turns are
>> logically equivalent to +a+, +a$, +a() whose semantics is totally unknown to
>> me.
>> Kind Regards,
>> Giovanni
>> Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:
>>  Per my action item from Thursday's telecon, following is more about my
>>> analysis and proposal for solving the "$has semantics problem" once
>>> and for all.
>>> *** ANALYSIS ***
>>> My first conclusion, on which I think we all agree, is that while +a+b
>>> might imply +a/+b and vice versa, that does not mean that: a) they
>>> have the same semantics, or b) that they identify the same XDI graph.
>>> Indeed, since one is an XDI subject and the other adds an XDI
>>> predicate, by definition they cannot identify the same XDI graph.
>>> To illustrate in English, most English verbs can be made into nouns by
>>> putting them in their infinitive form (“to be”, “to run”, “to shout”).
>>> But “run” and “to run”, while clearly linked semantically, do not have
>>> the same meaning. They cannot be interchanged with each other.
>>> Thus my second conclusion is that none of the three XDI graph examples
>>> we were discussing on our telecon two weeks ago were in fact
>>> equivalent. They were each different XDI graphs that had similar but
>>> not identical semantics.
>>> My third conclusion is that this means that we do in fact need clearly
>>> distinguished ways of semantically and syntactically of expressing
>>> that an XDI subject (e.g., +a) : 1) has a child subject (e.g., +a+b),
>>> and 2) has a predicate (e.g., +a/+b).
>>> My fourth conclusion was that this is the problem we have been
>>> struggling with for the last year: by using $has (with or without
>>> variants like $has$a) to express both parent subject/child subject
>>> relationships (+a+b) and subject/predicate relationships (+a/+b), we
>>> have wrapped ourself in a classic Gordian knot.
>>> *** PROPOSAL ***
>>> Once the problem was framed this way, it became much easier (for me
>>> anyway) to see that a clean solution would be to dedicate the $has
>>> predicate to only one semantic – either the parent subject/child
>>> subject relationship or the subject/predicate relationship. Of those
>>> two options, I strongly prefer dedicating the $has verb to expressing
>>> the subject/predicate relationship (+a/+b), since this is the simpler
>>> and more intuitive to understand, and mirrors the semantics of the $a
>>> predicate, which expresses predicate/object relationships.
>>> If $has is dedicated to that expression, then we simply need a
>>> different predicate (or set of predicates) to express parent
>>> subject/child subject relationships.
>>> For that, an obvious answer is to use XRI delimiters as predicates.
>>> This is intuitive since it is syntactically required to have a
>>> delimiter between XRI subsegments, and each of the six XRI delimiters
>>> (not counting parentheses for cross-references) has its own
>>> XRI-defined "semantics", e.g., * means reassignable identifier, !
>>> means persistent identifiers, etc.
>>> So to summarize using +a and +b notation:
>>> +a/$has/+b      <==>    +a/+b
>>> +a/+/+b <==>    +a+b
>>> +a/$/$b <==>    +a$b
>>> +a/=/=b <==>    +a=b
>>> +a/@/@b <==>    +a@b
>>> +a/*/*b <==>    +a*b
>>> +a/!/!b <==>    +a!b
>>> +a/()/(b)       <==>    +a(b)
>>> Semantically, it would still be true that +a+b/$is$a/+b (and the
>>> inverse that +b/$a/+a+b). And while +a+b would imply that +a/+b is
>>> valid, and vice versa, the existence of one in an XDI graph would not
>>> imply the existence of the other.
>>> So, if =example wanted to do a query to determine if +a+b and +a/+b
>>> both existed in an XDI graph, it would be:
>>> =example
>>>        $get
>>>                /
>>>                        +a
>>>                                $has
>>>                                        +b
>>>                                +
>>>                                        +b
>>> I have been trying out this proposal in sample XDI documents for the
>>> past week and it seems to work very well. Please consider and
>>> experiment with it and post your thoughts. We'll put it on the agenda
>>> for next week's telecon.
>>> =Drummond
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