- Favour use of argList methods that are more similar to dictionary
method names with the aim of reducing the cognitive load.
* Silently deprecate two-parameter get() method in favour of the
more familiar getOrDefault.
* Silently deprecate opt() method in favour of get()
These may be verbosely deprecated in future versions.
- remove writeGeometry() in favour of write() and make it pure virtual
so that all writers must explicitly deal with it.
- establish proxy extension at construction time and treated as an
invariant thereafter. This avoids potentially surprising changes in
behaviour when writing.