As we discussed on IRC: http://kessel.ilrt.bris.ac.uk/jruby/logs/2008-04-29.html#T22-19-16
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22:19:16 <mkrauskopf> Might be I would use --jdb for 'jdb' and --debug for '-J-Djruby.reflection=true -J-Djruby.compile.mode=OFF' mode
22:20:11 <mkrauskopf> --jdb will be likely used just by few developer of JRuby itself and its extensions. --debug would be better for users.
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Attaching patch.
This appears to have been already committed.