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One more thing before I forget - Jesse mentioned two interesting ideas to preserve upgradability: overlaying template files, and pre and post procedure callbacks (ability to execute code before and after a kernel proc is invoked without touching the kernel code). Something to think about for us perhaps.
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The message interceptor concept is naturally supported in xotcl. Having said that, I have thought of implementing this into the service contracts package -- eventhough, it won't be a solution as good as xotcl it is the closest it gets with the current suite of tools we are using.