To run Lemmings:

1. Locate the folder LemmingsSample, which should have in it files named
lemmings and Assets.  It's likely that the path to the folder will be
/Users/yourname/LemmingsSample or /Users/yourname/Downloads/LemmingsSample.

2. Open a Terminal window and type
        cd whateverThePathIs
where you should replace whateverThePathIs with the path to the
LemmingsSample folder.

3. Confirm you're in the right folder by typing
        ls
which should show you the Assets folder and the lemmings executable.

4. Type
        ./lemmings
to play the game.

Alternatively, in the folder Lemmings, you can move the Assets folder to
your home directory, then double-click on the lemmings executable file.

When you try to run the program under macOS Tahoe, a dialog box may come up
saying essentially that the system can't verify that the executable I'm giving
you isn't malicious.  Click Done.

If you trust that I'm not evil:
1. Click the Apple Menu icon in the upper left corner of your screen
2. Select System Settings.
3. In the left sidebar, scroll down and click Privacy & Security.
4. In the right frame, scroll down to the Security section.
5. Next to the "lemmings" that you see, click Open Anyway.
When you next run the program and get the dialog box, click the Open Anyway
button that now appears.  You'll be asked for an admin password.  If you
give it, from then on you'll be able to always run the program as usual,
with no further confirmation requests.
