# HISTORY, ITEM_TREE and other LOCATABLE at-codes

**Category:** [Implementers (archive)](https://discourse.openehr.org/c/implementers-archive/158)
**Created:** 2011-05-31 11:12 UTC
**Views:** 5
**Replies:** 1
**URL:** https://discourse.openehr.org/t/history-item-tree-and-other-locatable-at-codes/12725

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## Post #1 by @Justinas_Prelgauskas

Hello,

I've been looking through a very useful set of ADL 1.5 test archetypes related to Specialization feature of OpenEHR via "Knowledge2" repository ( [http://www.openehr.org/wsvn/knowledge2/TRUNK/archetypes/ADL_1.5_test/validity/specialisation](http://www.openehr.org/wsvn/knowledge2/TRUNK/archetypes/ADL_1.5_test/validity/specialisation) ).

I think some of archetype nodes lack "at"-codes.
For example OBSERVATION archetype "openEHR-EHR-OBSERVATION.spec_test_obs.v1" uses such types as HISTORY and ITEM_TREE.
However, both HISTORY and ITEM_TREE types are LOCATABLE.
Thus I think all such objects must have "at"-codes declared.
This must be true for all LOCATABLE objects, because they have mandatory property "archetype_node_id".
Or maybe I am missing something?

Regards,
Justinas Prelgauskas

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## Post #2 by @thomas.beale

Justinas,

the currently proposed rules for ADL1.5 (see the draft), see section 5.3.12 are that at-codes are optional. This allows many leaf and near-leaf level nodes not to have to have useless at-codes attached to them. The same goes for any node that is a single child of an attribute, like HISTORY etc. The motivation for this is to allow for simpler paths, and keep the size of archetypes down to what is needed.

The C_OBJECT.node_id property is mandatory in the AOM. What we do in the current implementations is that the node_id is set to the value 'unknown'. This is currently not properly specified in the spec, but is implemented in the [ADL workbench](http://www.openehr.org/svn/ref_impl_eiffel/TRUNK/apps/adl_workbench/doc/web/index.html).

hope this helps

- thomas

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**Canonical:** https://discourse.openehr.org/t/history-item-tree-and-other-locatable-at-codes/12725
**Original content:** https://discourse.openehr.org/t/history-item-tree-and-other-locatable-at-codes/12725