Some definitions of interest.
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop
l_member	Def (x  l) == i:. i<||l|| & x = l[i]  T
	Thm* T:Type, x:T, l:T List. (x  l)  Prop
nat	Def  == {i:| 0i }
	Thm*   Type
select	Def l[i] == hd(nth_tl(i;l))
	Thm* A:Type, l:A List, n:. 0n  n<||l||  l[n]  A

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