Some definitions of interest.
decidable	Def  Dec(P) == P  P
	Thm*  A:Prop. Dec(A)  Prop
not	Def  A == A  False
	Thm*  A:Prop. (A)  Prop
rev_implies	Def  P  Q == Q  P
	Thm*  A,B:Prop. (A  B)  Prop

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