Add a RecordReader to this collection. The id() of a RecordReader determines
where in the Tuple its
Create a new key common to all child RRs.
For all child RRs offering the key provided, obtain an iterator at that position
in the JoinCollecto
Obtain an iterator over the child RRs apropos of the value type ultimately
emitted from this join.
Return true if it is possible that this could emit more values.
Clone the key at the top of this RR into the given object.