Some definitions of interest.
assert	Def b == if b True else False fi
	Thm* b:. b  Prop
bnot	Def b == if b false else true fi
	Thm* b:. b
isl	Def isl(x) == InjCase(x; y. true; z. false)
	Thm* A,B:Type, x:A+B. isl(x)
outr	Def outr(x) == InjCase(x; y. "???"; z. z)
	Thm* A,B:Type, x:A+B. isl(x)  outr(x)  B

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