'Iterate through instances related to the given instance for
a particular \'cascade\', starting with this MapperProperty.
Return an iterator3-tuples (instance, mapper, state).
Note that the \'cascade\' collection on this MapperProperty is
checked first for the given type before cascade_iterator is called.
See PropertyLoader for the related instance implementation.'

def cascade_iterator(self, type_, state, visited_instances=None, halt_on=None):

    return iter(())
