Some definitions of interest.
decidable	Def Dec(P) == P  P
	Thm* A:Prop. Dec(A)  Prop
int_seg	Def {i..j} == {k:| i  k < j }
	Thm* m,n:. {m..n}  Type
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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