多层次的spef文件，来个大的例子

以下是一个多层次的 SPEF 文件的示例：

* SPEF 1.0
* Design: my_design
* Date: 2021/05/01
* Vendor: my_vendor
* Program: my_program
* Revision: 1.0

* Default capacitance unit: fF
* Default resistance unit: kOhm
* Default time unit: ps

* Top-level clock
* Clock name: clk
* Clock port: clk

* Top-level ports
* Port name: in1
* Port capacitance: 10
* Port resistance: 1
* Port name: in2
* Port capacitance: 15
* Port resistance: 1
* Port name: out1
* Port capacitance: 5
* Port resistance: 0.5

* Sub-circuit 1
* Instance name: sub1
* Instance capacitance: 20
* Instance resistance: 2
* Net name: sub1.in1
* Net capacitance: 5
* Net resistance: 0.5
* Net name: sub1.in2
* Net capacitance: 5
* Net resistance: 0.5
* Net name: sub1.out1
* Net capacitance: 10
* Net resistance: 1

* Sub-circuit 2
* Instance name: sub2
* Instance capacitance: 30
* Instance resistance: 3
* Net name: sub2.in1
* Net capacitance: 10
* Net resistance: 1
* Net name: sub2.in2
* Net capacitance: 15
* Net resistance: 1
* Net name: sub2.out1
* Net capacitance: 5
* Net resistance: 0.5

* Netlist
* Net name: in1
* Net capacitance: 10
* Net resistance: 1
* Net name: in2
* Net capacitance: 15
* Net resistance: 1
* Net name: sub1.out1
* Net capacitance: 10
* Net resistance: 1
* Net name: sub2.out1
* Net capacitance: 5
* Net resistance: 0.5
* Net name: out1
* Net capacitance: 5
* Net resistance: 0.5

在这个 SPEF 文件中，有顶层时钟、顶层端口、两个子电路、以及一个电路的净表。每个对象都有其自己的属性和子属性。例如，顶层端口具有名称、电容和电阻等属性，而子电路具有实例名称、电容和电阻、以及子电路中的净名称和净电容等属性。净表也具有净名称、电容和电阻等属性。这个 SPEF 文件的结构可以让电路设计工程师更好地理解和分析电路。