- Pass multiple child elements as positional args to Element(), instead
  of elem.append() or elem.extend().
- Return created elements immediately without a local variable.

- Write distinct tags for .v21.{Start,End}Period.
- Simplify writing DataKeySet and MemberSelection.
- Avoid use of typing.cast()

- Stash/unstash on AttributeSet, not ReportedAttribute.
- Generate concrete subclasses of ReportedAttribute.
- Resolve references in TargetIdentifiableObject.

…instead of str/bytes.

This was an erroneous inclusion, not part of the standard.

- Log non-identical members of DictLike.
- Log missing components in other ComponentList.

- Streamline .reader.v21._component_end()

Handle the case where the existing object is concrete, not an external
reference, but has no URN set (e.g. because not given in the source
message) by computing a URN.

- Distinguish NoWriterImplementation from NotImplementedError.
- When dispatching to a writer function for the parent class of an
  object, register that function for the child class directly.

Match on a string path fragment (same as test case ID), rather than a
complete path object.

Allow non-strict comparison if self.urn is None.

 - ReportStructure
 - IdentifiableObjectTarget
 - ReportPeriodTarget

Handle cases where objects are included explicitly within a
StructureMessage, but with isExternalReference="true", i.e. as
"stubs".