Some definitions of interest.
prev_nat_pair	Def  prev_nat_pair(xy) == xy/x,y. if x=0 <y-1,0> else <x-1,y+1> fi
	Thm*  prev_nat_pair  {xy:()| xy = <0,0>   }
eq_int	Def  i=j == if i=j true ; false fi
	Thm*  i,j:. (i=j)
nat	Def   == {i:| 0i }
	Thm*    Type
not	Def  A == A  False
	Thm*  A:Prop. (A)  Prop

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