A node that indexes incoming Tuples by their signatures as specified by a TupleMask. Notifies listeners about such
update events through the IndexerListener.
Signature tuples are created by transforming the update tuples using the mask. Tuples stored with the same signature
are grouped together. The group or a reduction thereof is retrievable.
This indexer will be updated whenever a Rete update is sent to the active node (or an equivalent time slot
allotted to it). The active node is typically the indexer itself, but it can be a different node such as its