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