The 'normal range' and 'is normal' functions in the reference model are implemented
as attributes to enable storing this information explicitly rather than in a functional manner.
The precedence of the attributes in DV_INTERVAL are:
1. lower or upper_unbounded take precedence over presence of
the lower or upper value
2. lower or upper_included only has meaning when the appropriate
value exists
matchString limits the values of the match attribute in the Term mapping class
archetypeNodeId pattern matches allowable atCodes and archetypeIds, for use in locatable
atCode pattern matches allowable atCodes for use in Activity ID on ISM_Transitions in ACTION
The reference model provides a means of restricting intervals to be of particular
types through the use of generic types. This is not available in XML schema - so
a set of restricted interval types is provided for this purpose.