Generic types in message specification

[treiher]

   generic
      Length_Type : <>;
      Value_Type : <>;
   type Length_Value is
      message
         Length : Length_Type
            then Value
               with Length => Length;
         Value : Value_Type;
      end message;

Instantiation of generic type:

   type Supported_Groups is new Length_Value (Supported_Groups_Length_Type, Named_Groups);

   type Example is
      message
         Supported_Groups : Supported_Groups;
      end message;