| 76 | 76 | The second relationship possible between archetypes is composition, allowing large data structures to be flexibly constrained via the hierarchical re-use of smaller archetypes. Composition is defined in terms of `slots' within an archetype. A slot is a point in an archetype structure where, instead of specifying an object type inline, a special allow_archetype constraint is used to specify other archetypes constraining that same type, that may be used at that point. For example, the archetype openEHR-EHR-SECTION.vital_signs.v1 defines a heading structure for headings to do with vital signs. It also defines as its items attribute value (i.e. what comes under the heading) a number of possible Observations; however, rather than defining these inline, it specifies an archetype slot in the form of constraints on Observation archetypes that are allowed at that point. The simplest kind of constraint is in terms of regular expressions on archetype identifiers. More complex constraints can be stated in terms of paths in other archetypes (for example exists(/some/path[at0005])). A slot thus defines a `chaining point' in terms of possible archetypes allowed or excluded at that point; limiting this to a single archetype is of course possible. Templates are used to choose which particular archetypes allowed at a slot will actually be used in a given circumstance. |
