Some definitions of interest.
decidable	Def Dec(P) == P  P
	Thm* A:Prop. Dec(A)  Prop
interleaving_occurence	Def interleaving_occurence(T;L1;L2;L;f1;f2) Def == ||L|| = ||L1||+||L2||   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, f1:(||L1||||L||), f2:(||L2||||L||). Thm* interleaving_occurence(T;L1;L2;L;f1;f2)  Prop
increasing	Def increasing(f;k) == i:(k-1). f(i)<f(i+1)
	Thm* k:, f:(k). increasing(f;k)  Prop
int_seg	Def {i..j} == {k:| i  k < j }
	Thm* m,n:. {m..n}  Type
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
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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