MAYBE

Problem:
 ge(x,0()) -> true()
 ge(0(),s(y)) -> false()
 ge(s(x),s(y)) -> ge(x,y)
 rev(x) -> if(x,eq(0(),length(x)),nil(),0(),length(x))
 if(x,true(),z,c,l) -> z
 if(x,false(),z,c,l) -> help(s(c),l,x,z)
 help(c,l,cons(x,y),z) -> if(append(y,cons(x,nil())),ge(c,l),cons(x,z),c,l)
 append(nil(),y) -> y
 append(cons(x,y),z) -> cons(x,append(y,z))
 length(nil()) -> 0()
 length(cons(x,y)) -> s(length(y))

Proof:
 Open