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.