0{}n0
1{x}n1
2{x,y}n2
3{x,y,p}n3
4{}n4
;
0(,,start?s)1
1(,,a?x)2
1(,,end!e)4
2(x>=0,,a?y)3
2(x<0,,error!x)1
3(y<0,,error!y)1
3((y>=0) _and (y-x<=2) _and (x-y<=2),[p:=(y-x)],ok!p )1
3((y>=0) _and ( (y-x>2) _or (x-y>2) ),,ok!x,y)1
;;
_E [ _true _U <a?x[x>=3]> _E [ _true _U <ok!p[p==2]> _true ]]