## 24. Some Useful List Functions

These functions all manipulate lists.

> `

>     > (append '(1 2 3) '(4 5 6))    ;concatenate lists
>     (1 2 3 4 5 6)
>     > (reverse '(1 2 3))            ;reverse the elements of a list
>     (3 2 1)
>     > (member 'a '(b d a c))        ;set membership -- returns the first tail
>     (A C)                           ;whose car is the desired element
>     > (find 'a '(b d a c))          ;another way to do set membership
>     A
>     > (find '(a b) '((a d) (a d e) (a b d e) ()) :test #'subsetp)
>     (A B D E)                       ;find is more flexible though
>     > (subsetp '(a b) '(a d e))     ;set containment
>     NIL
>     > (intersection '(a b c) '(b))  ;set intersection
>     (B)
>     > (union '(a) '(b))             ;set union
>     (A B)
>     > (set-difference '(a b) '(a))  ;set difference
>     (B)

> `

`Subsetp`, `intersection`, `union`, and `set-difference` all assume that each argument contains no duplicate elements -- `(subsetp '(a a) '(a b b))` is allowed to fail, for example.

`Find`, `subsetp`, `intersection`, `union`, and `set-difference` can all take a `:test` keyword argument; by default, they all use `eql`.