More blank tile information

The Blank Tile Information msgbox (accessible from the Select Tile window's right click context menu) seems to either say:

Quote:

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Tile is not blank.

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or

Quote:

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Tile is blank.

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followed by a 2x2 matrix of X's or -'s, but doesn't say what this matrix represents.

This msgbox is created by show_blank_tile in zq_tiles.cpp