place - a place; evaluated if an error is signaled.

datum-form - a form that evaluates to a datum. Evaluated each time an error is to be signaled, or not at all if no error is to be signaled.

argument-form - a form that evaluates to an argument. Evaluated each time an error is to be signaled, or not at all if no error is to be signaled.

datum, arguments - designators for a condition of default type error. (These designators are the result of evaluating datum-form and each of the argument-forms.)

The places are generalized references to data upon which test-form depends, whose values can be changed by the user in attempting to correct the error. Subforms of each place are only evaluated if an error is signaled, and might be re-evaluated if the error is re-signaled (after continuing without actually fixing the problem). The order of evaluation of the places is not specified; see Section 5.1.1.1 Evaluation of Subforms to Places. If a place form is supplied that produces more values than there are store variables, the extra values are ignored. If the supplied form produces fewer values than there are store variables, the missing values are set to nil.

 (setq x (make-array '(3 5) :initial-element 3))
 #2A((3 3 3 3 3) (3 3 3 3 3) (3 3 3 3 3))
 (setq y (make-array '(3 5) :initial-element 7))
 #2A((7 7 7 7 7) (7 7 7 7 7) (7 7 7 7 7))
 (defun matrix-multiply (a b)
   (let ((*print-array* nil))
     (assert (and (= (array-rank a) (array-rank b) 2)
                  (= (array-dimension a 1) (array-dimension b 0)))
             (a b)
             "Cannot multiply ~S by ~S." a b)
            (really-matrix-multiply a b)))   MATRIX-MULTIPLY
 (matrix-multiply x y)
 Correctable error in MATRIX-MULTIPLY: 
 Cannot multiply #<ARRAY ...> by #<ARRAY ...>.
 Restart options:
 1: You will be prompted for one or more new values.
 2: Top level.
 Debug> :continue 1
 Value for A: x
 Value for B: (make-array '(5 3) :initial-element 6)
 #2A((54 54 54 54 54)
       (54 54 54 54 54)
       (54 54 54 54 54)
       (54 54 54 54 54)
       (54 54 54 54 54))

 (defun double-safely (x) (assert (numberp x) (x)) (+ x x))
 (double-safely 4) 
 8
 
 (double-safely t)
 Correctable error in DOUBLE-SAFELY: The value of (NUMBERP X) must be non-NIL.
 Restart options:
 1: You will be prompted for one or more new values.
 2: Top level.
 Debug> :continue 1
 Value for X: 7
 14

The set of active condition handlers.