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TagBoxToBoxes

ErrorBox

FilledSmallSquareErrorBox[boxes] represents boxes that cannot be interpreted in input or output.

FilledSmallSquareErrorBox[boxes] typically displays as the raw form of boxes together with underlining that indicates parts that cannot be interpreted.

FilledSmallSquare See The Mathematica Book: Section 2.8.10.

FilledSmallSquare See also: TagBox, InterpretationBox, ToExpression.

TagBoxToBoxes