# An optional and an fixed pattern
PREFIX : <http://example.org/>

# Ensure an optional does not mask a later one.

# This query works unreordered because it has written out the needed UNION

SELECT * 
WHERE 
{
  :x :p ?v .
  OPTIONAL {:y ?p ?a } 
  OPTIONAL {:z ?p ?v }
}

## SELECT DISTINCT *
## {
##   :x :p ?v .
##     { OPTIONAL {:y ?p ?a }  OPTIONAL {:z ?p ?v } }
##   UNION
##     { OPTIONAL {:z ?p ?v }  OPTIONAL {:y ?p ?a } }
## }