(VAR x )
(RULES 
        a(f, 0) -> a(s, 0)
        a(d, 0) -> 0
        a(d, a(s, x)) -> a(s, a(s, a(d, a(p, a(s, x)))))
        a(f, a(s, x)) -> a(d, a(f, a(p, a(s, x))))
        a(p, a(s, x)) -> x
        
)