WOLFRAM LANGUAGE TUTORIAL
Pattern Sequences
In some cases you may need to specify pattern sequences that are more intricate than things like 
or x..; for such situations you can use PatternSequence[p1,p2,…].
Pattern sequences.
This defines a function with two or more arguments, grouping the first two.
Evaluate the function for different numbers of arguments.
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This picks out the longest run of the sequence
in the list.
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The empty sequence, PatternSequence[], is sometimes useful to specify an optional argument.
This picks out expressions with exactly one or two arguments.
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