Nested has been released on jedit plugin central, you can now install it via jedit's plugin manager

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In short, nested is a jedit plugin that lets you see when you edit files with nested languages, such as for example XML inside R (see this post to find out what the code is about) :

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Note, you need to install my R edit mode for jedit to recognize the xml within R.

21     <!-- deal with R4X inline XML -->
22     <SPAN DELEGATE="xml::MAIN" >
23       <BEGIN>'##((xml</BEGIN>
24       <END>'##xml))</END>
25     </SPAN> 
26