- Make nodetype overrideable.
- Add __getitem__ to SchemaNode.
- Capitalize node titles.
- Deal with empty strings in String, Boolean, and Float types.
- Introduce description; make title the same as name.
- Remove copy method from schemanode.
- Allow schema nodes to have titles.
- The term "structure" is too overloaded to use for schema purposes:
structure -> schema node.
- Make Sequence more like Tuple and Mapping (it uses a substructure
rather than a structure parameter to denote its type).
- Add __repr__ and copy methods to structure.
- Add accept_scalar flag to Sequence.