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| ANSI Common Lisp 12 Numbers 12.2 Dictionary of Numbers
12.2.33 incf, decf | Macro
  Syntax: incf place [delta-form] new-value decf place [delta-form] new-value Arguments and Values: place - a place. delta-form - a form; evaluated to produce a delta. The default is `1`. delta - a number. new-value - a number. Description: incf and decf are used for incrementing and decrementing the value of place, respectively. The delta is added to (in the case of incf) or subtracted from (in the case of decf) the number in place and the result is stored in place. Any necessary type conversions are performed automatically. For information about the evaluation of subforms of places, see Section 5.1.1.1 Evaluation of Subforms to Places. Examples:
       (setq n 0)
       (incf n)  1
       n  1
       (decf n 3)  -2
       n  -2
       (decf n -5)  3
       (decf n)  2
       (incf n 0.5)  2.5
       (decf n)  1.5
       n  1.5
Side Effects: Place is modified. See Also: +, -, 1+, 1-, setf Allegro CL Implementation Details: None.

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