An Accumulator is the processor for each section of a group, so there is one Accumulator for each group key.
Fatory to create accumulators.
An Aggregator is the processor for the whole result stream.
Accumulator that passes down every value of an expression
Syntax element and framework execution for custom aggregates.
The null aggregate (which can't be written in SPARQL) calculates nothing but does help remember the group key.
Registry of custom aggregates There is only a single global registry of aggregates - it affects parsing and parsing happens before Context or Execution makes sense.
Aggregate that does everything except the per-group aggregation that is needed for each operation
An Args is an ExprList with a flag for DISTINCT.
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