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syntax: <relation specification> ::=
              (<frame relation>*)]
              (<frame relation>*)]

        <frame relation> ::=
           ([:DOCUMENTATION <documentation>]
           [:EXPLANATION <explanation>]
             {<lisp-type: symbol> |
           [:NUMBER-RESTRICTION (<min><max>)]
           [:INITIAL-VALUE {<instance-name>|
              <evaluates-to-set> | 
              <lisp-type: T}]
           [:INITABLE {T | NIL}]
           [:READ-ONLY {T | NIL}]
           [:IF-UNDETERMINED <lisp-type: T>]
           [:READ-DEPENDENTS <lisp-type: T>]
           [:WRITE-DEPENDENTS <lisp-type: T>])

Warning: The explanation facility is not sup-
         ported, hence :documentation and
         :explanation are ignored.

With the relation specification it is possible to
determine the given feature or the given element more
exactly, when making the frame definition.
':documentation' and ':description' can contain
explanations for the user.
With ':value-restriction' it is possible to restrict
the range of the given attribute. Here can be used
lisp-types like string, number, list or a frame name.
If you use a frame name, then you can only use
instances of the corresponding frame with the instance
relation. With ':number-restriction' it is possible to
restrict the number of values, that can be entered at
the instance relation. The minimum and the maximum
number of values have to be defined.
':initial-value' describes a default value, that
should be entered during the initialization, if no
other value has been entered during the definition
of the instance.
With ':initable' it can be determined, whether the
initial value can be changed during the instanciation
or not. If T is used, it can be overwritten. If NIL
is used, the default value will always be used during
the initialization.
':read-only' avoids, that writing over the value,
which is given during the initialization is possible.
That means, that you can only get access to the value
in order to read it (for example with ASK).
':if-undetermined' describes, which actions have to
be executed, when the value of the instance relation
is unknown. For this a list of babylon-expressions has
to be set up, which will be executed in sequential
':read-dependents' give the possibility 
to handle read-access on a value more differentiated.
It is possible to send a message to other instances,
if there exists a read-access on the value. You have
to give an instance to the value, that has to have a
read-dependent. The message will be send to this
instance. In order to work out the message, it is
neccesary to describe a behavior with the name notify-
read-dependent, that treats the message for instances
of the corresponding class.
':write-dependents' function analogue to ':read-
dependents', except that here we are dealing with
writing accesses.

example: > (define-frame client
                 (:value-restriction symbol
              :if-undetermined (ask (match _x (start
              (tell [holidayinterest instance _x])))
              proposal (:value-restriction countries)
              destinations (:value-restriction
              :if-undetermined (ask [countries
              (tell [destinations instance

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