dynamics of functions

Dear Thomas,  

What about the following function?

iterate<-function(f,n,x){
  if(n==0) return(x)
  y<-x
  for(i in 1:n)y<-f(y)
  y
}
iterate(sqrt,3,256)

Thank you very much, this certainly helps.
I'm still curious, though, to know how to
write the expression of my function 
immediately as the argument f.
I can define it outside of the function

aap <- function(x) -x^3

and use it as argument

iterate(aap,3,256),

but I seem not to be clever enough
to write a function that receives
the following as input

iterate(-x^3,3,256)

Thanks again,

Tobias