At: shorten list 2 1 3
1. T: Type
2. L: T*
3. La: T*
4. Fin(T)
5. u: T
6. v: T*
7. La': T*
8. 
t:T. mem_f(T;t;v) 
mem_f(T;t;L) 
mem_f(T;t;La')
9. t:T. mem_f(T;t;La') mem_f(T;t;v)
10. ||La'||
1 
mem_f(T;hd(La');L)
11. mem_f(T;u;L)
12. ||La'||1
mem_f(T;hd(La');L)
By: BackThru -3
Generated subgoals:None
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