async def METHOD_NAME(websocket, watch_key):
    """
    Handle a connection from a spectator: watch an existing game.
    """
    # Find the Connect Four game.
    try:
        game, connected = WATCH[watch_key]
    except KeyError:
        await error(websocket, "Game not found.")
        return
    # Register to receive moves from this game.
    connected.add(websocket)
    try:
        # Send previous moves, in case the game already started.
        await replay(websocket, game)
        # Keep the connection open, but don't receive any messages.
        await websocket.wait_closed()
    finally:
        connected.remove(websocket)