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6.7.2.5 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 lst … obj
Scheme Procedure: append
Scheme Procedure: append! lst … obj
Scheme Procedure: append!
C Function: scm_append (lstlst)
C Function: scm_append_x (lstlst)

Return a list comprising all the elements of lists lstobj. If called with no arguments, return the empty list.

```(append '(x) '(y))          ⇒  (x y)
(append '(a) '(b c d))      ⇒  (a b c d)
(append '(a (b)) '((c)))    ⇒  (a (b) (c))
```

The last argument obj 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
```

`append` doesn’t modify the given lists, but the return may share structure with the final obj. `append!` is permitted, but not required, to modify the given lists to form its return.

For `scm_append` and `scm_append_x`, lstlst is a list of the list operands lstobj. That lstlst itself is not modified or used in the return.

Scheme Procedure: reverse lst
Scheme Procedure: reverse! lst [newtail]
C Function: scm_reverse (lst)
C Function: scm_reverse_x (lst, newtail)

Return a list comprising the elements of lst, in reverse order.

`reverse` constructs a new list. `reverse!` is permitted, but not required, to modify lst in constructing its return.

For `reverse!`, the optional newtail is appended to the result. newtail isn’t reversed, it simply becomes the list tail. For `scm_reverse_x`, the newtail parameter is mandatory, but can be `SCM_EOL` if no further tail is required.

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