(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
        
)