In all of the following operations, the elements
tmd:topicref, tmd:type,
tmd:roletype and tmd:player
allow a reference to a topic by either is unique object identifier
(using the oid attribute) or using a subject
identifier reference (using the psi attribute).
When the psi attribute is used, if no topic is
found that matches the specified subject identifer, a new, untyped
topic will be created and assigned that value as a subject identifier.
When the oid attribute is used, if no topic is
found in the topic map with the specified object identifier, an error
is raised.
Warning
Developers should be aware of the implicit topic creation by
using the psi attribute as described above. This
is a powerful tool for quick update of a topic map, but can create a
lot of unwanted topics if not used carefully.
