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Internal Occurrences (1)
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[No name]
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Unary association used to mark association types as being symmetrical.
Symmetric associations are associations like friend-of or neighbour-of
where X friend-of Y implies that also Y friend-of X. In Topic Maps these
are implemented as binary associations where the same role type is played
by both topics, as follows:
<pre>
friend-of(X : friend, Y : friend)
</pre>
This way, the association only has to be stated once to be applicable
in both directions. However, Ontopoly treats association types with
just a single role type as unary associations. Therefore, it's
necessary to state explicitly that these association types are
symmetric, and that is done with this unary association.
Usage:
<pre>
on:is-symmetric(friend-of : on:field-type)
</pre>
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