Some definitions of interest.
absval	Def |i| == if 0i i else -i fi
	Thm* x:. |x|
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop
nat	Def  == {i:| 0i }
	Thm*   Type
le	Def AB == B<A
	Thm* i,j:. (ij)  Prop

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