A terminal node for a query. Holds the tokens that reached it, to be read back by name.
There is no parameter list. A query is its conditions and its body.
Rete.Engine.query/3 lets the caller constrain any variable in :bind.
:index is the key sets Rete.Ruleset.index/2 declared, each sorted. A filter whose
keys are a superset of one of them reads a bucket instead of every match. It changes
speed and nothing else, so an empty list only means every filter scans.