private void FetchStreamStart()
{
// Initialize the simple key stack.
simpleKeys.Push(new SimpleKey());
// A simple key is allowed at the beginning of the stream.
simpleKeyAllowed = true;
// We have started.
streamStartProduced = true;
// Create the STREAM-START token and append it to the queue.
var mark = cursor.Mark();
tokens.Enqueue(new StreamStart(mark, mark));
}