Some definitions of interest.
compose_flips	Def compose_flips(L) == compose_list(map(i.(i, i+1);L))
	Thm* k:, L:(k-1) List. compose_flips(L)  kk
compose	Def (f o g)(x) == f(g(x))
	Thm* A,B,C:Type, f:(BC), g:(AB). f o g  AC
int_seg	Def {i..j} == {k:| i  k < j }
	Thm* m,n:. {m..n}  Type
nat	Def  == {i:| 0i }
	Thm*   Type
sum	Def sum(f(x) | x < k) == primrec(k;0;x,n. n+f(x))
	Thm* n:, f:(n). sum(f(x) | x < n)

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