Some definitions of interest.
factorial_via_iter	Def  k! == k!k
	Thm*  k:. k!
injection_type	Def  A inj B == {f:(AB)| Inj(A; B; f) }
	Thm*  A,B:Type. A inj B  Type
int_seg	Def  {i..j} == {k:| i  k < j }
	Thm*  m,n:. {m..n}  Type
one_one_corr_2	Def  A ~ B == f:(AB), g:(BA). InvFuns(A;B;f;g)
	Thm*  A,B:Type. (A ~ B)  Prop

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