data consistency tests with UNSAID and OPTIONAL

2004-12-21T14:56:01Z <SimonR> EricP suggests that constraint-checking
			      is a need for UNSAID; SteveH says this
			      can be done with OPTIONAL.
2004-12-21T14:56:55Z <ericP> s/constraint-checking/consistency checking/


Persuant to that I dreamed up a quick screw case (actually, it was the
first case I thought of):

Data: a1 b c1. 
      c1 d e. 
      a2 b c2.
      c2 b f.

UNSAID Query: SELECT ?a ?c
              WHERE ?a b ?c
              UNSAID ?c d e

Result: a2, c2

OPTIONAL Query: SELECT ?a ?c
                WHERE ?a b ?c
                OPTIONAL ?c d e

Result: a1, c1
        a2, c2

The only way to do this is to ask for a variable in place of e and use
test if ?e is equal to the URI <e>.

OPTIONAL Query: SELECT ?a ?c ?e
                WHERE ?a b ?c
                OPTIONAL ?c d ?e
                AND ?e op:URIequals <e>
-- 
-eric

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