Some definitions of interest.
int_seg	Def  {i..j} == {k:| i  k < j }
	Thm*  m,n:. {m..n}  Type
is_ident	Def  is_ident(A; f; u) == x:A. f(u,x) = x & f(x,u) = x
	Thm*  f:(AAA), u:A. is_ident(A; f; u)  Prop

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