| Some definitions of interest. |
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length | Def ||as|| == Case of as; nil 0 ; a.as' ||as'||+1 (recursive) |
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| Thm* A:Type, l:A List. ||l||  |
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| Thm* ||nil||  |
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nat | Def == {i: | 0 i } |
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| Thm* Type |
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nequal | Def a b T == a = b T |
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| Thm* A:Type, x,y:A. (x y) Prop |
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select | Def l[i] == hd(nth_tl(i;l)) |
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| Thm* A:Type, l:A List, n: . 0 n  n<||l||  l[n] A |