MAYBE

Problem:
 strict:
  f(act(),y) -> f(el(nact()),y)
  f(x,nact()) -> f(x,act())
  act() -> el(nact())
  l(el(x)) -> el(l(x))
  el(r(x)) -> r(el(x))
 weak:
  nact() -> l(nact())
  nact() -> r(nact())

Proof:
 Open