MAYBE
Trs:
 {                  app(app(filter(), f), nil()) -> nil(),
  app(app(filter(), f), app(app(cons(), x), xs)) -> app(app(app(if(), app(f, x)), app(app(cons(), x), app(app(filter(), f), xs))), app(app(filter(), f), xs)),
               app(app(app(if(), true()), x), y) -> x,
              app(app(app(if(), false()), x), y) -> y}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open