code: @Nonnull
    public TypeaheadDataset setPrefetch (@Nullable final ISimpleURL aURL)
    {
      return setPrefetch (aURL == null ? null : new TypeaheadPrefetch (aURL));
    }
docs: Can be a URL to a JSON file containing an array of datums or, if more
  configurability is needed, a prefetch options object.
  
  @param aURL
  URL to the JSON file. May be <code>null</code>.
  @return this