Some definitions of interest.
l_before	Def x before y  l == [x; y]  l
	Thm* T:Type, l:T List, x,y:T. x before y  l  Prop
no_repeats	Def no_repeats(T;l) == i,j:. i<||l||  j<||l||  i = j  l[i] = l[j]  T
	Thm* T:Type, l:T List. no_repeats(T;l)  Prop
sublist	Def L1  L2 Def == f:(||L1||||L2||).  Def == increasing(f;||L1||) & (j:||L1||. L1[j] = L2[(f(j))]  T)
	Thm* T:Type, L1,L2:T List. L1  L2  Prop

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