MAYBE
MAYBE
TRS:
 {
               is_empty(nil()) -> true(),
          is_empty(cons(x, l)) -> false(),
                hd(cons(x, l)) -> x,
                tl(cons(x, l)) -> l,
       ifappend(l1, l2, nil()) -> l2,
  ifappend(l1, l2, cons(x, l)) -> cons(x, append(l, l2)),
                append(l1, l2) -> ifappend(l1, l2, l1)
 }
 DUP: We consider a duplicating system.
  Trs:
   {
                 is_empty(nil()) -> true(),
            is_empty(cons(x, l)) -> false(),
                  hd(cons(x, l)) -> x,
                  tl(cons(x, l)) -> l,
         ifappend(l1, l2, nil()) -> l2,
    ifappend(l1, l2, cons(x, l)) -> cons(x, append(l, l2)),
                  append(l1, l2) -> ifappend(l1, l2, l1)
   }
  Fail