DocumentationMathematicaBuilt-in FunctionsProgrammingPattern Matching
_ or Blank[ ] is a pattern object that can stand for any Mathematica expression.
_h or Blank[h] can stand for any expression with head h.
The head h in _h cannot itself contain pattern objects.
See Section 2.3.1.
See also: Pattern, Optional, ForAll.
New in Version 1.