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I am running Build 93 of COW Express.

I discovered that changes made to an opening are saved automatically, which is how I messed up the London System.4MD, having experimented with it using the installed engine, and then exited without -- I thought -- saving it.

How can I retrieve the original London System.4MD file?

The best way might be to rename the London ebook that you have currently, assuming you want to keep the changes you made.

Then install that ebook again. Log in and download the installer if you've lost it.

You might also consider using color coding. If I remember correctly, the London II ebook has all of its moves coded with yellow. That way any move you add will not be color coded and thus can be compared to the original moves of the ebook.