Def  next_nat_pair(xy) == xy/x,y. if y=0 <0,x+1> else <x+1,y-1> fi

is mentioned by

Thm*  InvFuns(;{xy:()| xy = <0,0>   };next_nat_pair;prev_nat_pair)	[next_nat_pair_vs_prev_nat_pair]
Thm*  Inj(; ; next_nat_pair)	[next_nat_pair_inj]
Thm*  xy:(). <0,0> = next_nat_pair(xy)	[next_nat_pair_not_zeroes]
Def  nat_to_nat_pair(i) == next_nat_pair{i}(<0,0>)	[nat_to_nat_pair]

Try larger context: DiscrMathExt IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html