Some definitions of interest.
it_sum	Def it_sum(l) == if null(l) then 0 else head(l)+it_sum(tl(l)) fi   (recursive)
	Thm* l: List. it_sum(l)
nat	Def  == {i:| 0i }
	Thm*   Type
	Thm*   S
le	Def AB == B<A
	Thm* i,j:. (ij)  Prop

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