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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def retrieve(self, filter_expression=None, order_expression=None, slice_key=None):
 """ Retrieve entities from this cache, possibly after filtering, ordering and slicing. """

        ents = iter(self.__entities)
        if not filter_expression is None:
            ents = filter_expression(ents)
        if not order_expression is None:
            # Ordering always involves a copy and conversion to a list, so
            # we have to wrap in an iterator.
            ents = iter(order_expression(ents))
        if not slice_key is None:
            ents = islice(ents, slice_key.start, slice_key.stop)
        return ents