1.以设计为目标的程序库都必须帮助使用者完毕静止的设计。以实现使用者自己的constraints,而不是实现预先定义好的constraints.

2.Anything that can be done in more than one way should be identified and migrated from the 
   class to a policy

3.Design is choice. Most often, the struggle is not that there is no way to solve a design problem, but that 
 there are too many ways that apparently solve the problem. You must know which collection of solutions 
 solves the problem in a satisfactory manner.