PREFIX :    <http://example/>

## The nested optional example, rewritten to a form that is the same
## for the SPARQL algebra and the declarative semantics.
SELECT ?v ?w
{ 
    :x1 :p ?v .
    OPTIONAL { :x3 :q ?w }
    OPTIONAL { :x3 :q ?w  . :x2 :p ?v }
}
