    /*
     * call-seq:
     *  prefix
     *
     * Get the prefix for this namespace.  Returns +nil+ if there is no prefix.
     */
    static VALUE prefix(VALUE self)
    {
      xmlNsPtr ns;
      xmlDocPtr doc;

      Data_Get_Struct(self, xmlNs, ns);
      if(!ns->prefix) return Qnil;

      Data_Get_Struct(rb_iv_get(self, "@document"), xmlDoc, doc);

      return NOKOGIRI_STR_NEW2(ns->prefix);
    }