Some definitions of interest.
compose	Def (f o g)(x) == f(g(x))
	Thm* A,B,C:Type, f:(BC), g:(AB). f o g  AC
flip	Def (i, j)(x) == if x=i j ; x=j i else x fi
	Thm* k:, i,j:k. (i, j)  kk
int_seg	Def {i..j} == {k:| i  k < j }
	Thm* m,n:. {m..n}  Type

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