Some definitions of interest.
assert	Def b == if b True else False fi
	Thm* b:. b  Prop
bnot	Def b == if b false else true fi
	Thm* b:. b
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop

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