Some definitions of interest.
nat	Def   == {i:| 0i }
	Thm*    Type
nat_plus	Def   == {i:| 0<i }
	Thm*    Type
prime	Def  prime(a) == a = 0 & (a ~ 1) & (b,c:. a | bc  a | b  a | c)
	Thm*  a:. prime(a)  Prop

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