Add verify() without command to API

    }

    /**

     * Verify the completion of the exercise by executing a command and checking

     * if the output is as expected. If yes, the exercise is treated as completed,

     * if no, the exercise is treated as failed.

     * Verify the completion of the exercise by optionally executing a command and checking

     * if the output is as expected. If yes, the exercise is treated as solved, if no,

     * the user has to try again.

     * 

     * Used like this:

     * 

     * this.verify("cat botschaft").hasOutput("Der Junge sieht den Mann mit dem Fernglas.")

     * ```

     * 

     * verify() can also be called without a command, in which case no command is executed – instead

     * the last command entered by the user (if any) is used.

     * 

     * When the parameter passed to `hasOutput` is a string, the verification succeeds iff

     * this string is the first and only line of output of the command.

     * When the parameter passed to `hasOutput` is a list, the verification succeeds iff

     * @returns An object that has the method "hasOutput"

     */

    verify(command) {

        if(command) {

            runCommand(command)

        }

        const verifyHandler = this._verifyHandler

        return {

                    const terminalContents = getTerminalContents()

                    const latestEntry = terminalContents[terminalContents.length - 1]

                    if (latestEntry.input !== command) {

                    if (command && latestEntry.input !== command) {

                        throw new Error("Could not find expected input in terminal")

                    }