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BUILT-IN MATHEMATICA SYMBOL
BlankNullSequence 
(three 
characters) or BlankNullSequence[]
is a pattern object that can stand for any sequence of zero or more Mathematica expressions.
or BlankNullSequence[h]
can stand for any sequence of expressions, all of which have head h.
Details
- Blank sequences work slightly differently depending on whether or not the head of the expression in which they appear is a symbol with the attribute Flat.
- Consider matching the pattern
against the expression 
. If f is a symbol with attribute Flat, then the 
will be taken to stand for the expression 
. If f is not a symbol with attribute Flat, then 
will be taken to stand for the sequence of expressions 
, .... With a named pattern, such as 
, x can be used only as an element in an expression. The sequence of expressions 
, ... is "spliced in" to replace x, thereby usually increasing the length of the expression. - If
matches a sequence of length more than one, then the sequence will be represented by a Sequence object. In most uses of 
, however, the Sequence object will automatically be spliced into another expression, and will never appear explicitly.
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