Some definitions of interest.
assert	Def  b == if b True else False fi
	Thm*  b:. b  Prop
iff	Def  P  Q == (P  Q) & (P  Q)
	Thm*  A,B:Prop. (A  B)  Prop
nat	Def   == {i:| 0i }
	Thm*    Type

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