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