(* [cube n] is [n] cubed*)
let cube n =
  n *. n *. n
(* [sign n] is [n]'s sign *)
let sign n =
  if n = 0 then 0
  else if n > 1 then 1
  else -1
(* [area r] is area of circle with [r] radius
   assert ((area 1.) -. Float.pi < 1e-5)
 *)
let area r =
  Float.pi *. r *. r