title: OCaml empty global variable
body: <p>I'd like to know how do I define an empty! global variable of type <code>Hashtbl</code> in OCaml?
  I don't want to use <code>Hashtbl.create</code> because I don't know its initial size and I don't want to guess the initial size for performance reasons.</p>
  <p>Basically this <code>Hashtbl</code> variable will be assigned a real <code>Hashtbl</code> in a function and then this variable will be shared among other functions so I don't want to pass it around as an argument all the time hence I'd like it to be global.</p>