Change/Add Semantics of Content Elements

• Elements can use the definitonURL attribute to override default semantics (eg, infix, postfix)
• A separate formalism for defining semantics is part of the recommendation (but authors can use others)
• Authors can refer to other specifications through the csymbol:

<!-- reference to OpenMath formal syntax definition of Bessel function -->
<apply>
  <csymbol encoding="OpenMath" 
           definitionURL="http://www.openmath.org/cd/BesselFunctions.ocd">
    <msub><mi>J</mi><mn>0</mn></msub>
  </csymbol>
  <ci>y</ci>
</apply>