The Every operator applies an {@link Aggregator} or {@link Buffer} to every grouping.
Any number of Every instances may follow other Every, {@link GroupBy}, or {@link CoGroup} instances if they apply anAggregator, not a Buffer. If a Buffer, only one Every may follow a GroupBy or CoGroup.
Every operators create aggregate values for every grouping they encounter. This aggregate value is added to the current grouping Tuple.
In the case of a CoGroup, the grouping Tuple will be all the grouping keys from all joined streams, and if an "outer" type join is used, one value on the groupingTuple may be null.
Subsequent Every instances can continue to append values to the grouping Tuple. When an Each follows and Every, the Each applies its operation to the grouping Tuple (thus all child values in the grouping are discarded and only aggregate values are propagated).