Some definitions of interest.
decidable	Def Dec(P) == P  P
	Thm* A:Prop. Dec(A)  Prop
l_member	Def (x  l) == i:. i<||l|| & x = l[i]  T
	Thm* T:Type, x:T, l:T List. (x  l)  Prop
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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