problem([]==>x:pnat=>y:pnat=>minus(x,s(y))=pred(minus(x,y))in pnat,
dequantify then simplify then repeat intro,lambda(x,lambda(y,axiom)),
[problem([x:pnat,y:pnat,v0:pnat,v1:pnat]==>pred(v1)in pnat,
 repeat wfftac,axiom,
 [
 ]) ext _3906
]).
