CONSTRUCT SPARQL queries
CONSTRUCT queries need to be implemented.
These are almost identical to SELECT. There are 2 differences:
1. Only 3 elements in the selection, and they need not be variables (this is OK in Query, though SPARQL SELECT does not expect it).
2. Variables in the selection clause need to be constrained:
a. variable subjects ($subj) should have a "minus $subj <rdf:type> <rdfs:Literal> in <sys:type>" appended to the WHERE clause.
b. variable predicates should have a "and $pred <rdf:type> <mulgara:URIReference> in <sys:type>" appended to the WHERE clause.
Item (2) can be delayed, though it will result in a non-compliant CONSTRUCT query.