Adding 'Built-in' code

sounds structurally similar to onInit

Yes - although more advanced. I'd actually like it to be slightly 'hidden' - because I think the main use-case is to set it up, and then to have it working in the background without it being apparent.

How is the bootup code removed

E.setBootupCode("")?

what about multiple piece of boot up code?

I think, given the advanced nature, we'd ignore that. If there was a 'getter', you'd do:

E.setBootupCode(E.getBootupCode()+"print('Hello');")