editing MathML parts with Amaya

[Wolfgang Jeltsch <wolfgang@jeltsch.net>]

Hello,

I'm thinking about editing DocBook XML documents with embedded MathML using 
Emacs and nXML mode.  Alas, MathML is very verbose and therefore not easy to 
edit with a text editor.  So I'd like to edit the MathML parts with an 
external MathML editor, namely Amaya.  What I want is that if I place Emacs' 
cursor inside a MathML fragment and press a specific key combination, the 
whole MathML part is copied into a file (with some additional heading lines), 
Amaya is run on this file, and if Amaya has finished, the file's content 
(minus the heading lines) is written back into my Emacs buffer.

So far, I've written a script which basically puts standard input in a file, 
invokes Amaya and writes the resulting file content to standard output.  If I 
invoke this from Emacs via shell-command-on-buffer then after finishing 
Amaya, Emacs crashes with the message: "X protocol error: BadWindow (invalid 
Window parameter) on protocol request 25".

In addition, shell-command-on-buffer doesn't seem to be the right thing 
because it places the command's output in * Shell Command Output * instead of 
replacing the text in the file's buffer.

So how do I filter parts of a buffer through a shell command so that the 
respective part of the buffer is replaced by the command's output?  Can 
somebody of you imagine why the above error message appears?  Is there a 
better way to invoke X clients for editing parts of a document?  How can I 
tell Emacs to search for the boundaries of a MathML part the cursor is in, so 
that I don't have to mark the whole MathML fragment before editing it?

Thanks a lot for your answers.

Best wishes,
Wolfgang