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