IDL provides two types of unions: encapsulated and nonencapsulated. An IDL union must be discriminated. In an encapsulated union, the discriminator is part of the union. In a nonencapsulated union, the discriminator is not part of the union.
The following type_specifier can be used to indicate either kind of union.
union [tag]
A definition of the union identified by tag must appear elsewhere in the interface definition.
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