CLHS: Section 6.1.8.1

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## 6.1.8.1 Examples of clause grouping

    ;; Group conditional clauses.
     (loop for i in '(1 324 2345 323 2 4 235 252)
           when (oddp i)
             do (print i)
             and collect i into odd-numbers
             and do (terpri)
           else                              ; I is even.
             collect i into even-numbers
           finally
             (return (values odd-numbers even-numbers)))
    >>  1
    >>
    >>  2345
    >>
    >>  323
    >>
    >>  235
    =>  (1 2345 323 235), (324 2 4 252)

    ;; Collect numbers larger than 3.
     (loop for i in '(1 2 3 4 5 6)
           when (and (> i 3) i)
           collect it)                      ; IT refers to (and (> i 3) i).
    =>  (4 5 6)

    ;; Find a number in a list.
     (loop for i in '(1 2 3 4 5 6)
           when (and (> i 3) i)
           return it)
    =>  4

    ;; The above example is similar to the following one.
     (loop for i in '(1 2 3 4 5 6)
           thereis (and (> i 3) i))
    =>  4

    ;; Nest conditional clauses.
     (let ((list '(0 3.0 apple 4 5 9.8 orange banana)))
       (loop for i in list
             when (numberp i)
               when (floatp i)
                 collect i into float-numbers
               else                                  ; Not (floatp i)
                 collect i into other-numbers
             else                                    ; Not (numberp i)
               when (symbolp i)
                 collect i into symbol-list
               else                                  ; Not (symbolp i)
                 do (error "found a funny value in list ~S, value ~S~%" list i)
             finally (return (values float-numbers other-numbers symbol-list))))
    =>  (3.0 9.8), (0 4 5), (APPLE ORANGE BANANA)

    ;; Without the END preposition, the last AND would apply to the
    ;; inner IF rather than the outer one.
     (loop for x from 0 to 3
           do (print x)
           if (zerop (mod x 2))
             do (princ " a")
              and if (zerop (floor x 2))
                    do (princ " b")
                    end
              and do (princ " c"))
    >>  0  a b c
    >>  1
    >>  2  a c
    >>  3
    =>  NIL

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