Some definitions of interest.
decidable	Def Dec(P) == P  P
	Thm* A:Prop. Dec(A)  Prop
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop
rev_implies	Def P  Q == Q  P
	Thm* A,B:Prop. (A  B)  Prop

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