(VAR x xs ys )
(RULES 
        f(true,xs) -> f(isList(xs),append(cons(a,nil),xs))
        isList(nil) -> true
        isList(cons(x,xs)) -> isList(xs)
        append(nil,ys) -> ys
        append(cons(x,xs),ys) -> cons(x,append(xs,ys))
        
)