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Special descriptor for class __provides__
The descriptor caches the implementedBy info, so that
we can get declarations for objects without instance-specific
interfaces a bit quicker.
For example:
  >>> from zope.interface import Interface
  >>> class IFooFactory(Interface):
  ...     pass
  >>> class IFoo(Interface):
  ...     pass
  >>> class C(object):
  ...     implements(IFoo)
  ...     classProvides(IFooFactory)
  >>> [i.getName() for i in C.__provides__]
  ['IFooFactory']
  >>> [i.getName() for i in C().__provides__]
  ['IFoo']
 
    Overrides:
        AuthBase.__provides__
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