Wraps an operation in the projection that asks whether it has any solution at all.
SPARQL has neither a sub-ASK nor an empty projection, so the question is written as a projection onto a
single variable bound to a constant: one solution comes out exactly when the operation has one, whatever
that operation binds
(proof this works).
The variable it binds leaves the projection, so the context coins it: two of them sharing a name would
share a join key, and a MINUS decides compatibility on exactly the variables its two sides share.
Wraps an operation in the projection that asks whether it has any solution at all.
SPARQL has neither a sub-ASK nor an empty projection, so the question is written as a projection onto a single variable bound to a constant: one solution comes out exactly when the operation has one, whatever that operation binds (proof this works).
The variable it binds leaves the projection, so the context coins it: two of them sharing a name would share a join key, and a MINUS decides compatibility on exactly the variables its two sides share.