MAYBE

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

Proof:
 Open