istoryBack() {
                    if (this.history.length === 0) {
                        return;
                    } else if (this.historyIndex === -1) {
                        // we're not currently navigating the history; store
                        // the current command and then look at the end of our
                        // history buffer
                        this.beforeHistoryNav = this.input;
                        this.historyIndex = this.history.length - 1;
                    } else