csymbol as a function space
<math xmlns="http://www.w3.org/1998/Math/MathML"> <apply> <csymbol definitionURL="http://www.example.org/function_spaces.html#my_def" encoding="text">BigK</csymbol> <ci>x</ci> <ci>y</ci> </apply> </math>
Author: MathML 2 Recomendation