TRS:
 { sum(0()) -> 0(),
  sum(s(x)) -> +(sqr(s(x)), sum(x)),
     sqr(x) -> *(x, x),
  sum(s(x)) -> +(*(s(x), s(x)), sum(x))}
 POP*:
  Precedence:
  sqr > *, 
  sum > +, 
  sum > sqr, 
  sum > *
  empty
  
Normal:
   pi(sqr) = [1], pi(sum) = [1]
  
Safe:
   pi(*) = [1,2], pi(s) = [1], pi(+) = [1,2]
  
Predicative System:
   {  sum(0();) -> 0(),
    sum(s(;x);) -> +(;sqr(s(;x);),sum(x;)),
        sqr(x;) -> *(;x,x),
    sum(s(;x);) -> +(;*(;s(;x),s(;x)),sum(x;))}
  Qed