Selection based on attribute value

Given the following XML document fragment:

  <Portal:ScreenPreferences>
   <Portal:Screen primary="true">
    <Portal:screenTitle>Portal</Portal:screenTitle>
    <Portal:screenColor>#FFCC66</Portal:screenColor>
   </Portal:Screen>
   <Portal:Screen>
    <Portal:screenTitle>Services</Portal:screenTitle>
    <Portal:screenColor>#CC6600</Portal:screenColor>
   </Portal:Screen>
   <Portal:Screen>
    <Portal:screenTitle>Intranet</Portal:screenTitle>
    <Portal:screenColor>#0099CC</Portal:screenColor>
   </Portal:Screen>
   <Portal:Screen>
    <Portal:screenTitle>Control Panel</Portal:screenTitle>
    <Portal:screenColor>#CC9933</Portal:screenColor>
   </Portal:Screen>
   <Portal:Screen>
    <Portal:screenTitle>E-Commerce</Portal:screenTitle>
    <Portal:screenColor>#339900</Portal:screenColor>
   </Portal:Screen>
   <Portal:Screen>
    <Portal:screenTitle>Preferences</Portal:screenTitle>
    <Portal:screenColor>#CCEECC</Portal:screenColor>
   </Portal:Screen>
  </Portal:ScreenPreferences>

I want (in my XSL stylesheet) to be able to get the color and title for the
<Portal:Screen> element where the attribute 'primary' has the value true.  I
will take care of all the IMPLIED rules and so forth in the DTD; I just am
not sure how to select this data... any help is appreciated.

Thanks,
Brett


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