CLHS: Section 6.1.6.1

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## 6.1.6.1 Examples of WHEN clause

    ;; Signal an exceptional condition.
     (loop for item in '(1 2 3 a 4 5)
           when (not (numberp item))
            return (cerror "enter new value" "non-numeric value: ~s" item))
    Error: non-numeric value: A

    ;; The previous example is equivalent to the following one.
     (loop for item in '(1 2 3 a 4 5)
           when (not (numberp item))
            do (return
                (cerror "Enter new value" "non-numeric value: ~s" item)))
    Error: non-numeric value: A

    ;; This example parses a simple printed string representation from
    ;; BUFFER (which is itself a string) and returns the index of the
    ;; closing double-quote character.
     (let ((buffer "\"a\" \"b\""))
       (loop initially (unless (char= (char buffer 0) #\")
                         (loop-finish))
             for i of-type fixnum from 1 below (length (the string buffer))
             when (char= (char buffer i) #\")
              return i))
    =>  2

    ;; The collected value is returned.
     (loop for i from 1 to 10
           when (> i 5)
             collect i
           finally (prin1 'got-here))
    >>  GOT-HERE
    =>  (6 7 8 9 10)

    ;; Return both the count of collected numbers and the numbers.
     (loop for i from 1 to 10
           when (> i 5)
             collect i into number-list
             and count i into number-count
           finally (return (values number-count number-list)))
    =>  5, (6 7 8 9 10)

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