Gocept.reference

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- Require newer ``zope.app.generations`` version to get rid of
dependency on ``zope.app.zopeappgenerations``.

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- Validation of ``gocept.reference.field.Set`` now allows both
``InstrumentedSet`` and ``set`` in field validation, as both
variants occur.

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- ``zope.app.form`` breaks encapsulation of the fields by using the
``_type`` attribute to convert form values to field values. Using
``InstrumentedSet`` as ``_type`` was a bad idea, as only the
reference collection knows how to instantiate an
``InstrumentedSet``. Now the trick is done on validation where the
``_type`` gets set to ``InstrumentedSet`` temporarily.

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- Take advantage of the simpler zope package dependencies achieved at the Grok
cave sprint in January 2009.

- Added zope.schema field ``gocept.reference.field.Set`` which has the
internally used InstrumentedSet as field type, so validation does
not fail.

- gocept.reference 0.5.2 had a consistency bug: Causing a TypeError by
trying to assign a non-collection to a ReferenceCollection attribute
would break integrity enforcement for that attribute while keeping
its previously assigned value.

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- Fixed: When upgrading gocept.reference to version 0.5.1, a
duplication error was raised.

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- Made sure that the reference manager is installed using
zope.app.generations before other packages depending on
gocept.reference.