Conditional statements in DTD?

Hi All,
 
I am certain you cant use conditional statements within the DTD file so is it possible to make an attribute valid only if a parent element had an attribute set to something e.g.
 
The 'StepTwo' element is valid as the 'allow' attribute in 'StepOne' is set to 'yes'
 
< root>
   < StepOne allow="yes">
      < StepTwo playing="hard"
   < / StepOne
< / root>
 
The 'StepTwo' element is invalid as the 'allow' attribute in 'StepOne' is set to 'no'
 
< root>
   < StepOne allow="no">
      < StepTwo playing="hard">
   < / StepOne
< / root>
 
The 'StepTwo' element is valid because the 'allow' attribute is set to 'no' and there is no attribute for Step Two.

 
< root>
   < StepOne allow="no">
      < StepTwo>
   < / StepOne
< / root>

Surely things like this could be rather common?


 
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