Currently docbook 5 requires a targetdoc attribute on
olink. But there is one case where targetdoc is not
required: when you are processing olinks between
content modules in a single book. This practice has
been in place for some time because targetdoc was not
required in DocBook4, and the stylesheets have
supported it for some time.
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See the TC minutes from September 2006:
http://lists.oasis-open.org/archives/docbook-
tc/200609/msg00004.html