If we come across a record we know has children of (potential)
interest, but where the record itself is boring, but where other
records may care about where this one lives, we create one
of these. It allows us to get at the children, and track where on
disk this is, but not much else.
Anything done using this should quite quickly be transitioned to its
own proper record class!
Create a new holder for a boring record with children, but with
position dependent characteristics
Return the value we were given at creation
Write the contents of the record back, so it can be written