TRS:
 {   d(x) -> e(u(x)),
  d(u(x)) -> c(x),
  c(u(x)) -> b(x),
  v(e(x)) -> x,
  b(u(x)) -> a(e(x))}
 POP*:
  Quasi-Precedence:
  d ~ c > u ~ e, 
  d ~ c > b, 
  b > a
  u ~ e
  
Normal:
   pi(b) = [1], pi(c) = [1], pi(d) = [1]
  
Safe:
   pi(a) = [1], pi(u) = [1], pi(e) = [1]
  
Predicative System:
   {    d(x;) -> e(;u(;x)),
    d(u(;x);) -> c(x;),
    c(u(;x);) -> b(x;),
    v(e(;x);) -> x,
    b(u(;x);) -> a(;e(;x))}
  Qed