Url.CurrentThis module can be used to handle the Url associated to the current document.
Because the fragment of the Url for the current document can change dynamically, we use a functional value here.
val get : unit -> url optionget () returns a value of type url with fields reflecting the state of the current Url.
val set : url -> unitset u replaces the current Url for u. WARNING: Causes the document to change.