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To-do: archetypes.schemaextender
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 o Test, test, test! We need to find out when this breaks. ;)

Investigate:
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 o Explicit ordering of adapters (e.g. by interface specificity)

 o At the moment, users must subclass fields and mix in ExtensionField.
   We could either ship with or generate on-the-fly a field type for
   each of the standard Archetypes fields. However, we should have tests
   covering all fields in that case.
