/// <summary>
 /// Find all extensions with the given uri.
 /// </summary>
 /// <param name="extendable"></param>
 /// <param name="uri"></param>
 /// <returns>The list of extensions with a matching uri, or empty list if none were found.</returns>
 /// <remarks>If multiple extensions with the same uri are found, this function returns the first modifier extensions,
 /// otherwise the first normal extension.</remarks>
 public static IEnumerable <Extension> GetExtensions(this IModifierExtendable extendable, string uri)
 {
     return(extendable.AllExtensions().Where(ext => ext.Url == uri));
 }
 public static bool HasExtensions(this IModifierExtendable extendable)
 {
     return(!extendable.Extension.IsNullOrEmpty() && !extendable.ModifierExtension.IsNullOrEmpty());
 }