[XXE] How to make macro execute conditionally on Linux and Windows?

[Denis]

I'm just diving into the xxe commands - a newbie question: I expected the 
following
configuration code in customize.exe to open emacs on Linux, notepad on Windows:

<command name="editXMLSource">
     <macro>
       <sequence>
         <command name="XXE.save" parameter="[ifNeeded]" />
         <choice>
           <command name="run" parameter='c:\windows\system32\notepad.exe "%D"' 
/>
           <command name="run" parameter='emacs "%D"' />
         </choice>
         <command name="XXE.open" parameter="[reopenIfNewer]" />

       </sequence>
     </macro>
   </command>

   <binding>
     <keyPressed code="ESCAPE" />
     <charTyped char="e" />
     <command name="editXMLSource" />
   </binding>

According to the doc, "the choice element will execute the first alternative 
which
can be executed". Instead, the macro fails if the first choice fails: So as 
written,
the macro works only on Windows. On Linux it fails with this error: Execution 
failed
with exit status 127. /bin/sh: c:windowssystem32notepade.exe: command not found.

What am I misunderstanding?