Some definitions of interest.
factorial_via_iter	Def  k! == k!k
	Thm*  k:. k!
factorial_tail_via_iter	Def  k!m ==  i:{k-m..k}. i+1
	Thm*  m,k:. k!m
nat	Def   == {i:| 0i }
	Thm*    Type
le	Def  AB == B<A
	Thm*  i,j:. (ij)  Prop

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