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