MAYBE

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

Proof:
 Open