Some definitions of interest.
ndiff	Def  a -- b == imax(a-b;0)
	Thm*  a,b:. (a -- b)
	Thm*  a,b:. (a -- b)
imax	Def  imax(a;b) == if ab b else a fi
	Thm*  a,b:. imax(a;b)
nat	Def   == {i:| 0i }
	Thm*    Type

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