CLHS: Section 6.1.4.2

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## 6.1.4.2 Examples of ALWAYS, NEVER, and THEREIS clauses

    ;; Make sure I is always less than 11 (two ways).
    ;; The FOR construct terminates these loops.
     (loop for i from 0 to 10
           always (< i 11))
    =>  T
     (loop for i from 0 to 10
           never (> i 11))
    =>  T

    ;; If I exceeds 10 return I; otherwise, return NIL.
    ;; The THEREIS construct terminates this loop.
     (loop for i from 0
           thereis (when (> i 10) i) )
    =>  11

    ;;; The FINALLY clause is not evaluated in these examples.
     (loop for i from 0 to 10
           always (< i 9)
           finally (print "you won't see this"))
    =>  NIL
     (loop never t
           finally (print "you won't see this"))
    =>  NIL
     (loop thereis "Here is my value"
           finally (print "you won't see this"))
    =>  "Here is my value"

    ;; The FOR construct terminates this loop, so the FINALLY clause
    ;; is evaluated.
     (loop for i from 1 to 10
           thereis (> i 11)
           finally (prin1 'got-here))
    >>  GOT-HERE
    =>  NIL

    ;; If this code could be used to find a counterexample to Fermat's
    ;; last theorem, it would still not return the value of the
    ;; counterexample because all of the THEREIS clauses in this example
    ;; only return T.  But if Fermat is right, that won't matter
    ;; because this won't terminate.

     (loop for z upfrom 2
           thereis
             (loop for n upfrom 3 below (log z 2)
                   thereis
                     (loop for x below z
                           thereis
                             (loop for y below z
                                   thereis (= (+ (expt x n) (expt y n))
                                              (expt z n))))))

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