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