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

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

  return NOKOGIRI_STR_NEW2(ns->prefix);
}