Some definitions of interest.
arb	Def arb(T) == InjCase(lem(T); x. x, "uu")
	Thm* T:S. arb(T)  T
singleton	Def {a:T} == {x:T| x = a  T }
	Thm* T:Type, a:T. {a:T}  Type
stype	Def S == {T:Type| x:T. True }
	Thm* S  Type{2}

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