Some definitions of interest.
factorial_tail_via_iter	Def  k!m ==  i:{k-m..k}. i+1
	Thm*  m,k:. k!m
nat	Def   == {i:| 0i }
	Thm*    Type
not	Def  A == A  False
	Thm*  A:Prop. (A)  Prop

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