Some definitions of interest.
absval	Def |i| == if 0i i else -i fi
	Thm* x:. |x|
int_nzero	Def  == {i:| i  0 }
	Thm*   Type
nat_plus	Def  == {i:| 0<i }
	Thm*   Type
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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