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(librep.info)Void Variables

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Void Variables
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   A variable which has no value is said to be "void", attempting to
reference the value of such a symbol will result in an error. It is
possible for the most recent binding of a variable to be void even
though the inactive bindings may have values.

 - Function: boundp variable
     Returns true if the symbol VARIABLE has a value.

 - Function: makunbound variable
     This function makes the current binding of the symbol VARIABLE be
     void, then returns VARIABLE.

          (setq foo 42)
              => 42
          foo
              => 42
          (boundp 'foo)
              => t
          (makunbound 'foo)
              => foo
          (boundp 'foo)
              => ()
          foo
              error--> Value as variable is void: foo