[TF-PP] Duplicates and simple property path expressions.

In
http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2010Feb/0018.html

Rob Vesse observes that "no duplicates" is at odds with the expansion of 
a simple path (a term we have already decided to remove) into a triple 
pattern because there is an implicit projection going on.

   {?x foaf:knows{2} ?y}
is not quite the same as
   { ?x foaf:knows ?z . ?z foaf:knows ?y }
when there are acylic components/
e.g.:

:a foaf:knows :b .
:a foaf:knows :c .
:b foaf:knows :d .
:c foaf:knows :d .

because ?z is projected away.

We can specify either way:

+ emphasis that certain property paths are the same as the triple 
expansion form, and not have the striuct no duplicates rule (this 
reintroduces the simple property paths concept) or

+ note, and provide an example, that they are not exactly equivalent.

I prefer the latter - keep the "no duplicates" situation.

Note this is related to "[TF-PP] cycles - simple or a walk"

http://lists.w3.org/Archives/Public/public-rdf-dawg/2010JanMar/0455.html

	Andy

	Andy