a policy decision tree的用法和样例：
- Overrule a policy decision.
- Administrators of a lower policy level cannot loosen a policy decision made on a higher level, but they can tighten policy as much as they want.
- Such a policy decision would certainly lift the property industry, mainly through reviving speculation rather than genuine final demand, I believe.
- In the policy-based network management system of IETF, the Policy Enforcement Point (PEP) configurates policy through a policy distribution mechanism of the Policy Decision Point (PDP).