def parse_player(player):
    """Parse an input string for the first player
    Args:
        player (string): String to be parsed (eg, X)
    Returns:
        (string/None): The player if valid or None
    """
    if player.lower() == "x":
        return "X"
    elif player.lower() == "o":
        return "O"
    else:
        print("Player must be X or O")
        return None