Who Cites col?
col	Def Collection(T) == TProp
	Thm* T:Type{i'}. col{i:l}(T) Type{i'}
col_map	Def < f(x) | x c > (y) == x:T. x c & y = f(x) T'
	Thm* T,T':Type, f:(TT'), c:Collection(T). < f(x) | x c > Collection(T')
col_member	Def x c == c(x)
	Thm* T:Type, x:T, c:Collection(T). x c 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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