MAYBE

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

Proof:
 Open