A SeqPosition for sequences that need more than a Pos int for a position.
For such sequences, a Pos int is an index into a PositionManager,
which manages a table of ExtPositions, which may contain more state
than a regular SeqPosition does.
Get a position int "cookie" for this SeqPosition.
The result can be passed to AbstractSequence's getPosNext(int),
createRelativePos, and other methods.
By default this is the value of ipos, but for sequences that need emore
state than an ipos for efficient position, we use a PositionManager index.
So this gets over-ridden in ExtPosition.
Tests whether the position pair has the "isAfter" property.
I.e. if something is inserted at the position, will
the iterator end up being after the new data?
A toNext() or next() command should set isAfter() to true;
a toPrevious or previous command should set isAfter() to false.