Def  prev_nat_pair(xy) == xy/x,y. if x=0 <y-1,0> 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]

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