The current model of DV_EHR_URI contains hard-wired
Boolean functions called target_is_ehr, target_is_section etc.
These were intended to support the constraining of DV_EHR_URIs in
particular archetypes, by allowing the archetype definer to
specify a URI which pointed somewhere in the EHR, to a Compostion,
Section, Entry, or other specific type of node in the data.

However, with the use of Xpath-style paths now in use (see EHR IM)
it is no longer appropriate to have these functions built into this
type.