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based on an earlier version of the Wolfram Language.
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MatchQNot (!)

ValueQ

FilledSmallSquareValueQ[expr] gives True if a value has been defined for expr, and gives False otherwise.

FilledSmallSquareValueQ has attribute HoldFirst.

FilledSmallSquareValueQ gives False only if expr would not change if it were to be entered as Mathematica input.

FilledSmallSquare See The Mathematica Book: Section 2.3.5.

FilledSmallSquare See also: Information.

Further Examples

MatchQNot (!)