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External link sets

linkbase: an XML document whose primary purpose is to contain XLink link elements defining out-of-line links.

Linkbases are located either

An external linkset is a link with role xlink:external-linkset, e.g.
  <xlink:extended role="xlink:external-linkset">
    <xlink:locator href="..."/>
When processing a document with an external linkset, all links in the linkbase being referenced to should be fetched and processed.

(Both extended and simple links can have role attributes - link roles have nothing to do with resource roles.)