Some definitions of interest.
int_nzero	Def  == {i:| i  0 }
	Thm*   Type
nat_plus	Def  == {i:| 0<i }
	Thm*   Type
nequal	Def a  b  T == a = b  T
	Thm* A:Type, x,y:A. (x  y)  Prop
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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