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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 rename_db_ref(stmts_in, ns_from, ns_to, **kwargs):
 """Rename an entry in the db_refs of each Agent. This is particularly useful when old Statements in pickle files need to be updated after a namespace was changed such as 'BE' to 'FPLX'. Parameters stmts_in : list[indra.statements.Statement] A list of statements whose Agents' db_refs need to be changed ns_from : str The namespace identifier to replace ns_to : str The namespace identifier to replace to save : Optional[str] The name of a pickle file to save the results (stmts_out) into. Returns ------- stmts_out : list[indra.statements.Statement] A list of Statements with Agents' db_refs changed. """

    logger.info('Remapping "%s" to "%s" in db_refs on %d statements...' %
                (ns_from, ns_to, len(stmts_in)))
    stmts_out = [deepcopy(st) for st in stmts_in]
    for stmt in stmts_out:
        for agent in stmt.agent_list():
            if agent is not None and ns_from in agent.db_refs:
                agent.db_refs[ns_to] = agent.db_refs.pop(ns_from)
    dump_pkl = kwargs.get('save')
    if dump_pkl:
        dump_statements(stmts_out, dump_pkl)
    return stmts_out