I don't have much time to answer (being on vacation with
    grandchildren), but to enable debug messages -that is 
(debug arguments.... )
    expressions- and assertions -that is 
(assert_msg
      string test .... ) expressions-
    you need 
two things:
    
    
* when compiling your MELT code, you need to get
      the quicklybuilt or debugnoline flavor of the
      module (if using the default optimized flavor, debug
      facilities are disabled)
      
      * when running your MELT module (of quicklybuilt
      flavor) you need to pass -fplugin-arg-melt-debug option
    
    I hope this helps.
    
    Merry Christmas to all.
    
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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*** opinions {are only mine, sont seulement les miennes} ***