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 22.2.5 Append and Reverse
 -------------------------
 
 `append' and `append!' are used to concatenate two or more lists in
 order to form a new list.  `reverse' and `reverse!' return lists with
 the same elements as their arguments, but in reverse order.  The
 procedure variants with an `!' directly modify the pairs which form the
 list, whereas the other procedures create new pairs.  This is why you
 should be careful when using the side-effecting variants.
 
  -- Scheme Procedure: append . args
  -- C Function: scm_append (args)
      Return a list consisting of the elements the lists passed as
      arguments.
           (append '(x) '(y))          =>  (x y)
           (append '(a) '(b c d))      =>  (a b c d)
           (append '(a (b)) '((c)))    =>  (a (b) (c))
      The resulting list is always newly allocated, except that it
      shares structure with the last list argument.  The last argument
      may actually be any object; an improper list results if the last
      argument is not a proper list.
           (append '(a b) '(c . d))    =>  (a b c . d)
           (append '() 'a)             =>  a
 
  -- Scheme Procedure: append! . lists
  -- C Function: scm_append_x (lists)
      A destructive version of `append' ( Pairs and lists
      (r5rs)Pairs and lists.).  The cdr field of each list's final pair
      is changed to point to the head of the next list, so no consing is
      performed.  Return a pointer to the mutated list.
 
  -- Scheme Procedure: reverse lst
  -- C Function: scm_reverse (lst)
      Return a new list that contains the elements of LST but in reverse
      order.
 
  -- Scheme Procedure: reverse! lst [new_tail]
  -- C Function: scm_reverse_x (lst, new_tail)
      A destructive version of `reverse' ( Pairs and lists
      (r5rs)Pairs and lists.).  The cdr of each cell in LST is modified
      to point to the previous list element.  Return a pointer to the
      head of the reversed list.
 
      Caveat: because the list is modified in place, the tail of the
      original list now becomes its head, and the head of the original
      list now becomes the tail.  Therefore, the LST symbol to which the
      head of the original list was bound now points to the tail.  To
      ensure that the head of the modified list is not lost, it is wise
      to save the return value of `reverse!'
 

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