identifies the datatype of an attribute value, either by referencing an item in an externally defined datatype library, or by
pointing to a TEI-defined data specificationthe identifier used for this datatype specificationthe name of a datatype in the list provided by
XML Schemas: Part 2: Datatypesa pointer to a datatype defined in some datatype librarysupplies a string representing a regular expression providing additional constraints
on the strings used to represent values of this datatypedatatype used for attributes taking an integer value between 0 and 99
Only one of the attributes key, name, and ref may be used.