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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 cli(env, context_id, subnet_id, subnet_type):
 """Remove a subnet from an IPSEC tunnel context. The subnet id to remove must be specified. Remote subnets are deleted upon removal from a tunnel context. A separate configuration request should be made to realize changes on network devices. """

    manager = SoftLayer.IPSECManager(env.client)
    # ensure context can be retrieved by given id
    manager.get_tunnel_context(context_id)

    succeeded = False
    if subnet_type == 'internal':
        succeeded = manager.remove_internal_subnet(context_id, subnet_id)
    elif subnet_type == 'remote':
        succeeded = manager.remove_remote_subnet(context_id, subnet_id)
    elif subnet_type == 'service':
        succeeded = manager.remove_service_subnet(context_id, subnet_id)

    if succeeded:
        env.out('Removed {} subnet #{}'.format(subnet_type, subnet_id))
    else:
        raise CLIHalt('Failed to remove {} subnet #{}'
                      .format(subnet_type, subnet_id))