    /*
     * call-seq:
     *  register_ns(prefix, uri)
     *
     * Register the namespace with +prefix+ and +uri+.
     */
    static VALUE register_ns(VALUE self, VALUE prefix, VALUE uri)
    {
      xmlXPathContextPtr ctx;
      Data_Get_Struct(self, xmlXPathContext, ctx);

      xmlXPathRegisterNs( ctx,
                          (const xmlChar *)StringValuePtr(prefix),
                          (const xmlChar *)StringValuePtr(uri)
      );
      return self;
    }