Add PendingUpdate support

[GoogleCodeExporter]

This enhancement is to provide support for the "transactional" semantics in the 
XQuery Update Facility.

We need a PendingUpdate<N> interface.  It has a <N>ode, an operation, and a 
list of arguments.

We need an UpdatableModel<N> interface.  It has methods the same as, or similar 
to, MutableModel, but MutableModel is immediate effect, and UpdatableModel 
produces the Iterable<PendingUpdate<N>> instead.  UpdatableModel adds 
mergeUpdates and applyUpdates.

We might want UpdatableCursor.  This may complicate things (because there could 
then be update lists in two places, and a question of synchronization).

We need to modify MutableContext to return UpdatableModel.  We should do so in 
such a way that a specific bridge can support MutableModel only, UpdatableModel 
only, or both, or neither.

We need this in order to fully support XQuery Update Facility, which provides 
this "transactional" semantic.

Original issue reported on code.google.com by aale...@gmail.com on 3 Dec 2010 at 8:50

[GoogleCodeExporter]

Lower priority. Interesting for adaptation to databases and such; less 
important for a solid runtime API over tree models.

Original comment by aale...@gmail.com on 11 May 2011 at 3:48

• Added labels: Priority-Low
• Removed labels: Priority-Medium

[GoogleCodeExporter]

Definitely a worry-later.

Original comment by aale...@gmail.com on 26 Jul 2012 at 7:50

• Added labels: Milestone-Future