MAYBE

Problem:
 strict:
  a(x1) -> b(c(x1))
  a(b(x1)) -> b(a(x1))
  a(c(x1)) -> c(a(x1))
 weak:
  d(c(x1)) -> d(a(x1))
  d(a(x1)) -> d(c(b(x1)))

Proof:
 Open