Added a quadraticSpline enumerated value to the interpolate attributes of the <independentVarPts> and <independentVarRef> elements in response to a request from Geoff Brian of DSTO; fixed typo in cubicSpline attribute string. Added reference to Wikipedia article on spline interpolation.

Added a classification attribute to the <reference> element; added a date attribute to the <modificationRecord> element, per suggestions by Geoff Brian of DSTO.

Added two-D and three-D ungridded table examples and figures; corrected typo on ungridded table definition syntax (thanks to Dr. Peter Grant of U. Toronto's UTIAS and Geoff Brian of Australia's DSTO).

Reintroduced <!ENTITY> to include MathML2 DTD (complete) in the body of this DTD. This entity definition quietly went away in version 1.6 due to a misunderstanding of proper way to include external DTDs; it is reintroduced to assist validating parsers.

Added a <description> sub-element to the <provenance> element, so the provenance entry can contain more information about change justification documents; made <provenance> or <provenanceRef> acceptable sub-elements to <variableDef> and <checkData> elements after a request from Geoff Brian of DSTO.