As to your second question: Yes, Bill Joy did that in 1976 when he added a visual mode to his line editor ex that was itself based on ed. The mode was called vi (for "visual") and updated the screen as you typed ex commands preceded by a colon. All vi descendants have this feature, including Vim. :-)
I think it was in ed already. The POSIX ed spec says: "Any character other than <space> or <newline> can be used instead of a slash to delimit the RE and the replacement."
The Yoneda Lemma is really some kind of "fundamental theorem" of elementary category theory. Then there are Freyd's Adjoint Functor Theorem, Kan extensions and probably others I'm forgetting...
> We already know that a significant majority of the loans in CLOs have weak covenants that offer investors only minimal legal protection; in industry parlance, they are “cov lite.” The holders of leveraged loans will thus be fortunate to get pennies on the dollar as companies default—nothing close to the 70 cents that has been standard in the past.