Who Cites append?
append	Def as @ bs == Case of as; nil bs ; a.as' a.(as' @ bs) (recursive)
	Thm* T:Type, as,bs:T List. (as @ bs) T List
discrete_equality	Def {T=} == {eq:(TT)| x,y:T. (eq(x,y)) x = y }
	Thm* T:Type. {T=} Type
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
rev_implies	Def P Q == Q P
	Thm* A,B:Prop. (A B) Prop

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