private Streamer<?> resolveStreamer(Aggregation aggregation, Aggregation.Step step) {
Streamer<?> streamer;
AggregationFunction<?> aggFunction = (AggregationFunction<?>)functions.get(aggregation.functionIdent());
if (aggFunction == null) {
throw new ResourceUnknownException("unknown aggregation function");
}
switch (step) {
case ITER:
assert aggFunction.info().ident().argumentTypes().size() == 1;
streamer = aggFunction.info().ident().argumentTypes().get(0).streamer();