lispdoc - results for decf

(decf place &optional (delta 1) &environment env)

Function: The first argument is some location holding a number. This number is decremented by the second argument, DELTA, which defaults to 1.

Example:

(defun account-withdraw (account amt)
  "Make a withdrawal from this account."
  (if (<= amt (account-balance account))
      (decf (account-balance account) amt)
      'insufficient-funds))

Mentioned in:
CLtL2 - 12.4. Arithmetic Operations
CLtL2 - 7.11. Dynamic Non-Local Exits
CLtL2 - 7.2. Generalized Variables
HyperSpec - Macro INCF, DECF
On Lisp - CLOS and Lisp
PCL - basic math
PCL - defmethod
PCL - good object oriented design
PCL - other ways to modify places