Class FirstNBuffer will return the first N tuples seen in a given grouping. After the tuples are returned the Buffer stops iterating the arguments unlike the {@link cascading.operation.aggregator.First}{@link cascading.operation.Aggregator} which by contract sees all the values in the grouping.
By default it returns one Tuple.
Order can be controlled through the prior {@link cascading.pipe.GroupBy} or {@link cascading.pipe.CoGroup}pipes.
This class is used by {@link cascading.pipe.assembly.Unique}.