S9 LIB  (define-structure <name> <slot> ...)  ==>  unspecific

DEFINE-STRUCTURE creates a new type, which is a sub-type of the vector,
and defines a set of procedures for creating objects of the new type,
accessing its slots, and checking for its type.

<Name> is the name of the new type. Each <slot> defines a slot of the
new type. It must be have one of the following forms:

      <slot-name>
      (<slot-name>)
      (<slot-name> <initial-value>)

<Slot-name> must be a symbol and <initial-value> may be any value.
When an <initial-value> is specified, the corresponding slot will
be filled with that value whenever a new instance of the structure
is created. When the value is omitted, it defaults to an unspecific
value. <Slot-name> is equal to (<slot-name>).

(define-structure <type> <slot-1> ... <slot-N>) will expand to
definitions of the following procedures:

(make-<type> object ...) creates a new object of the type <type> and
initializes its slots with the values specified in DEFINE-STRUCTURE.
When some OBJECTs are given, they will replace the default values of
the first slots of the new <type> object. The number of OBJECTs
passed to MAKE-<TYPE> must not be larger than the number of slots
of <type>.

(<type>? x) is a predicate checking whether X has the type <type>.

(<type>-assert caller object) asserts that OBJECT is of the type
<type>. When the assertion holds, it returns an unspecific value.
Otherwise, it prints an error message. CALLER is a symbol that
will be reported as the source of the error (typically the
procedure calling <type>-assert).

(<type>-copy object) creates an exact (shallow) copy an object of
the given type and returns it.

(<type>-<slot-1> x) evaluates to the value stored in slot <slot-1>
of X. When X is not of the type <type>, an error will be signalled.
(<type>-<slot-N> x) does the same, but accesses <slot-N>.

(<type>-set-<slot-1>! x v) changes the value stored in slot <slot-1>
of X to V. When X is not of the type <type>, an error will be signalled.
(<type>-set-<slot-N>! x v) does the same, but changes <slot-N>.

(begin
  (define-structure point (x 0) (y 0) (color #f))
  (let ((p (make-point)))
    (point-set-color! p 'yellow)
    (list (point? p)
          (point-color p))))               ==>  (#t yellow)
