Extending subject schemes

The <schemeref> element provides a mechanism for extending a subject scheme. This makes it possible to add new relationships to existing subjects and extend enumerations of controlled values.

The <schemeref> element provides a reference to another subject scheme map. Typically, the referenced subject-scheme map defines a base set of controlled values that are extended by the current subject-scheme map. The values in the referenced subject-scheme map are merged with the values in the current subject-scheme map; the result is equivalent to specifying all of the values in a single subject scheme map.

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