/*
 * 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;
}