Some definitions of interest.
assert	Def b == if b True else False fi
	Thm* b:. b  Prop
choose	Def @x:T. P(x) == InjCase(lem({x:T| P(x) }); x. x, arb(T))
	Thm* T:S, P:(TType). (@x:T. P(x))  T
label	Def t  ...$L == t
stype	Def S == {T:Type| x:T. True }
	Thm* S  Type{2}

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