Some definitions of interest.
bif	Def bif(b; bx.x(bx); by.y(by)) == if b x(*) else y(x.x) fi
	Thm* A:Type, b:, x:(bA), y:((b)A). bif(b; bx.x(bx); by.y(by))  A
lt_int	Def i<j == if i<j true ; false fi
	Thm* i,j:. (i<j)
nat	Def  == {i:| 0i }
	Thm*   Type
	Thm*   S

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