OverStrike control character

[Ken Whistler]

On 6/9/2020 3:51 PM, abraham gross via Unicode wrote:
> What do yall think about adding an OverStrike control character?
Not gonna happen.
>
> Theres historical precedence of having such a control character. The famous Symbolics Space Cadet keyboard had such a key, and many typewriters relied on the its functionality (e.g. in order to make a ⟨!⟩ you had to type ⟨'<BS>.⟩ in most typewriters up until the mid 1900s)
And actually U+0008 BACKSPACE (i.e. BS) has been in the Unicode Standard 
for 30 years now. If people were going to implement characters a la the 
1980's (and earlier) backspace and overstrike mode, they had the 
character they needed for that already. It is a clue about how 
implementations work with characters and fonts these days that nobody is 
rushing out to implement overstriking with U+0008, even though it is 
there for the taking.
>
> The programming language APL also heavily relied on the overstrike control character, so many systems in the 80s had the character including Lisp machines.

Another telling example. Unicode 1.0 in 1990 included U+2300 APL COMPOSE 
OPERATOR, which was intended precisely for the APL overstrike 
functionality. It was *removed* in the big merger that resulted in 
Unicode 1.1, in part because the APL community itself was more 
interested in getting the actual composed operators into the encoding, 
rather than depending on archaic sequences that reflected the 
limitations of 7- and 8-bit character encodings and associated 
keyboards. Hence, all the combined APL operators now seen in Unicode at 
U+2336 .. U+2379.

There is some (very) limited use of the concept of an overstruck 
compositor in the Unicode Standard, but the concept is limited to 
specific scripts and is very constrained. The obvious example is U+13436 
EGYPTIAN HIEROGLYPH OVERLAY MIDDLE. That is used as part of a syntax for 
constructing complete Egyptian hieroglyph quadrats. But the critical 
distinction is that that format control is part of a complex syntax used 
by a modern font technology that maps sequences of hieroglyphs into 
ligatures and/or using complex positioning and resizing rules.

Nobody implements fonts these days that will just "back up" "one space" 
and "overstrike" a new character. Well, possibly outside the context of 
Societies for Deliberate Anachronism busy implementing emulations of 
long dead technology, I suppose.

--Ken