open System
open Option

type Safearray<'T> =
  struct
   val A:Option<'T> [],
   new(n) = {A = Array.create n None;} // create array of None instead of null
   
  end;;


type KVpair<'A,'B> =   // key-value pair
  struct
    val mutable key:'A   
    val mutable value:'B
    new(x,y) = {key=x; value=y;} // can have constructor of struct
    override this.ToString() = sprintf "%A:%A" this.key this.value
  end;;