Extracts direct variable assignments from the EXTEND chain at the top of an operation, keeping only the
ones whose expression is a Literal or a NamedNode.
Parameters
op: Operation
The operation to search
Returns Record<string,RDF.Term>
a record mapping variable names to their assigned terms
Extracts direct variable assignments from the EXTEND chain at the top of an operation, keeping only the ones whose expression is a Literal or a NamedNode.