Understanding the ROS{} type (from X.880)

Frank Derks frank.derks at PHILIPS.COM
Wed Jan 24 13:06:11 EST 2001


When reading through H.450.1, I came across the definition for the ROS type
(as defined in X.880):

ROS {InvokeId:InvokeIdSet, OPERATION:Invokable, OPERATION:Returnable) ::= CHOICE
{
    invoke       [1]  Invoke {{InvokeIdSet},{Invokable}},
    returnResult [2]  ReturnResult {{Returnable}},
    returnError  [3]  ReturnError {{Errors{{Returnable}}}},
    reject       [4]  Reject
}

I think I understand the lines with the "invoke" and the "returnResult"
(apart from the fact that two curly braces are used in the latter, but I
assume that this is just the way things are with the notation). Both
Invoke and ReturnResult are parameterized sequences and their respective
parameters are simply being "passed in".

The line that follows beats me, however. How should I read:

"{{Errors{{Returnable}}}}"

First of all, I would expect a parameter of type "ERROR" to be "passed in",
but it would seem that "Errors" is the name of the parameter for the
parameterized "ReturnError" sequence and "Returnable" is of the "OPERATION"
type. So what does that line actually mean?

Regards,

Frank

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