(VAR x1 )
(RULES 
        a(a(d(d(x1)))) -> d(d(b(b(x1))))
        a(a(x1)) -> b(b(b(b(b(b(x1))))))
        b(b(d(d(b(b(x1)))))) -> a(a(c(c(x1))))
        c(c(x1)) -> d(d(x1))
        
)