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  • p ? test is a pattern object that stands for any expression which matches p, and on which the application of test gives True.
  • Any result for test [ pval ] other than True is taken to signify failure.
  • Example: _?NumberQ represents a number of any type. The _ matches any expression, and ?NumberQ restricts to any expression which gives True on application of the number test NumberQ.
  • The operator ? has a high precedence. Thus _^_?t is _^(_?t) not (_^_)?t.
  • In a form such as __? test every element in the sequence matched by __ must yield True when test is applied.
  • See the Mathematica book: Section 2.3.5.
  • See also: Condition.

    Further Examples

    Here are two definitions.



    The definition for p1 applies only when x is manifestly a number.



    The definition for p2 is more liberal.