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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 rpc_start( working_dir, port, subdomain_index=None, thread=True ):
 """ Start the global RPC server thread Returns the RPC server thread """

    rpc_srv = BlockstackdRPCServer( working_dir, port, subdomain_index=subdomain_index )
    log.debug("Starting RPC on port {}".format(port))

    if thread:
        rpc_srv.start()

    return rpc_srv