Thm* 
T:Type, l:(T List
). ||l|| 
0 ; a.as' ||as'||+1 (recursive)
Thm* A:Type, l:A*. ||l||
Thm* ||nil||
| length_p |
Def ||l|| == ||l||
Thm* |
| length |
Def ||as|| == Case of as; nil 0 ; a.as' ||as'||+1 (recursive)
Thm*
Thm* ||nil|| |
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