(VAR x1 )
(RULES 
        f(x1) -> n(c(n(a(x1))))
        c(f(x1)) -> f(n(a(c(x1))))
        n(a(x1)) -> c(x1)
        c(c(x1)) -> c(x1)
        n(s(x1)) -> f(s(s(x1)))
        n(f(x1)) -> f(n(x1))
        
)