Corrected link to [Jackson04] paper.

Added discrete enumeration selection to interpolate attribute of <independentVarPts> and <independentVarRef> elements at the suggestion of Geoff Brian, DSTO.

Added a section and a <variableDef> example on extending the MathML-2 function set with atan2.

Removed all xns attributes from examples.

Amplified that it is a good practice to provide <variableDef>s in sorted sequence.