Fixed a bug that causes save file loss

The bug is caused by:

1. Close the game inside a level. In that case, some unlock progress is missing. But the line progress is still there. Visit locked levels should unlock them.
2. Draw on a board of large width (7-3-8 and 8-?-3). In that case, the game crashes on saving and save file is lost. If you can find a file named "understand_old.sav" in your game folder, rename it to understand.sav. It should restore your progress before yesterday's update.

If you want to restore more progress, you may modify the save file by:

First, make a backup. (Never forget to make a backup when modifying is what I just learnt) Open it with notepad. The level names within the first pair of square brackets are your solved levels. ? levels are named as EX instead. (For example, 1-? as EX-1) Just add what you need. An example: [["1-1","1-2","1-3","1-4","1-5","1-6","1-7","1-8","1-9","1-10","EX-1"], (followed by your last visited level, version number of the game, saved lines, and many others)

Sorry for the trouble. My bad programming and testing habits should be blamed. I'm making a test tool to avoid future mistakes like this.

To show my apologies, a free chapter update will be available in next month.