Y erased symbols should disappear

[hcs64]

This may be a matter of disagreement, but I think that when a Y erases a symbol, that symbol should disappear as in the game. Or at least it should be marked up in some to indicate it has been eliminated.

[gracenotes]

This is a great idea. There is a major (slow-going) refactoring I'm doing affecting interactions between Ys and stars which will actually invalidate many existing puzzles in order to make Ys erase things, rather than just correct errors.

The change to the validation logic will make this validate rather than error out:

Making the symbol disappear is a UI change that would be included in this.