why comments are outside of core? (is there any "rule of thumb"...)

[Robert Balejik]

I noticed that comments are outside the core directory, what is the rule to place components...outside the core directory?

[Arunoda Susiripala]

That's just to show the module system. Check this for some patterns for creating modules.
See: https://kadirahq.github.io/mantra/#sec-Appendix-Organizing-Modules