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Blank (_)BlankNullSequence (___)

BlankSequence

FilledSmallSquare__ (two _ characters) or BlankSequence[ ] is a pattern object that can stand for any sequence of one or more Mathematica expressions.

FilledSmallSquare__h or BlankSequence[h] can stand for any sequence of one or more expressions, all of which have head h.

FilledSmallSquare See notes for BlankNullSequence.

FilledSmallSquare See The Mathematica Book: Section 2.3.8.

Further Examples

Blank (_)BlankNullSequence (___)