Parameter Entity References for Namespace Prefixes

• Must use double indirection

• <!ENTITY % mathml-colon     ''>
  <!ENTITY % mathml-prefix    ''>
  
  <!ENTITY % mathml-exp       '%mathml-prefix;%mathml-colon;exp' >
  <!ENTITY % mathml-abs       '%mathml-prefix;%mathml-colon;abs' >
  <!ENTITY % mathml-arg       '%mathml-prefix;%mathml-colon;arg' >
  <!ENTITY % mathml-real      '%mathml-prefix;%mathml-colon;real' >
  <!ENTITY % mathml-imaginary '%mathml-prefix;%mathml-colon;imaginary' >
  
  <!ELEMENT %mathml-real; (%mathml-exp; | %mathml-abs;)*>

• Only works in external DTD subset