grammar rover;

public <command> = forwards {forwards} 
                 | backwards {backwards}
                 | back {backwards}
                 | reverse {backwards}
                 | stop {stop}
                 | left {left}
                 | right {right}
                 | up {up}
                 | down {down}
                 | whoa {stop}
                 | halt {stop}
                 | spin {left}
                 | go {forwards};

public <bye> = Good bye {bye} | So long {bye} | Shut down {bye};
