> On a serious note, libary methods, like that method, ought to be fast,

Every method in the image is a library method, so what do you mean by
"like" that method?  Below, you said can't base on the situation(s)
it's currently used, so are there any methods in the image which
*shouldn't* be so desperately optimized?  I'm just trying to figure
out your basis for saying "ought" above, about fast but not
readability.  Are those not the personal priorities of an expert
developer?  What about the priorities of the students and users in the
Dynabook dream?

> because you can't know in what situation they will be used. Saying that it's
> not even used is therefore not a valid reason.

That's only true for the very lowest-level methods.  Generally, it's
better to design and code for what's known, and NOT try to code for
the future / unknown.  IF and when that future comes, it'd be easy
enough for you to deal with any performance issues at THAT time.  In
the meantime, we "ought"  :) to keep as much beautiful minimal
elegance as we can.