Some definitions of interest.
biject	Def Bij(A; B; f) == Inj(A; B; f) & Surj(A; B; f)
	Thm* A,B:Type, f:(AB). Bij(A; B; f)  Prop
compose_flips	Def compose_flips(L) == compose_list(map(i.(i, i+1);L))
	Thm* k:, L:(k-1) List. compose_flips(L)  kk
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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