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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