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