Some definitions of interest.
interleaving	Def interleaving(T;L1;L2;L) Def == ||L|| = ||L1||+||L2||   & disjoint_sublists(T;L1;L2;L)
disjoint_sublists	Def disjoint_sublists(T;L1;L2;L) Def == f1:(||L1||||L||), f2:(||L2||||L||). Def == increasing(f1;||L1||) & (j:||L1||. L1[j] = L[(f1(j))]  T) Def == & increasing(f2;||L2||) & (j:||L2||. L2[j] = L[(f2(j))]  T) Def == & (j1:||L1||, j2:||L2||. f1(j1) = f2(j2))
	Thm* T:Type, L1,L2,L:T List. disjoint_sublists(T;L1;L2;L)  Prop
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop
length	Def ||as|| == Case of as; nil  0 ; a.as'  ||as'||+1  (recursive)
	Thm* A:Type, l:A List. ||l||
	Thm* ||nil||
nat	Def  == {i:| 0i }
	Thm*   Type

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