Illustratation of prefix interaction details

Here's a pathological example of when prefixes and QNames can be confusing.

As I read the current grammar, this is legal.  Only
CONSTRUCT and DESCRIBE can use QName in the
  http://www.w3.org/TR/rdf-sparql-query/#prod-sparql-ReportFormat
between the two PrefixDecl allowed in the Query term.

Additionally FROM can also use QNames in 
  http://www.w3.org/TR/rdf-sparql-query/#prod-sparql-FromClause
since it indirectly uses 
  http://www.w3.org/TR/rdf-sparql-query/#prod-sparql-URI
which can be a QName.

So the a:bar are different URIs below.  (A DESCRIBE example would be similar)

The a:abc is a source URI or graph name.

Dave

------
PREFIX a: <http://example.org/ns1#>
PREFIX a: <http://example.org/ns2#>
CONSTRUCT
  (a:foo a:bar "blah")
PREFIX a: <http://example.org/ns3#>
FROM a:abc
WHERE
  (?x a:bar ?y)