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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 get(self, cond=None, doc_id=None, eid=None):
 """ Get exactly one document specified by a query or and ID. Returns ``None`` if the document doesn't exist :param cond: the condition to check against :type cond: Query :param doc_id: the document's ID :returns: the document or None :rtype: Element | None """

        doc_id = _get_doc_id(doc_id, eid)

        # Cannot use process_elements here because we want to return a
        # specific document

        if doc_id is not None:
            # Document specified by ID
            return self._read().get(doc_id, None)

        # Document specified by condition
        for doc in self.all():
            if cond(doc):
                return doc