At 17:18 -0700 7/6/1999, Jonathan Coxhead wrote (among other things):
[snip]
>PRESENTATION SUGGESTION COMPOSE
>------------ ---------- -------
>
> Requests that characters be overstruck. Applies to the 2 characters
>on each side, like a "binary operator".
>
> Although seemingly simple, this introduces a whole set of problems.
>What is the difference between following a character with a COMBINING
>ENCLOSING CIRCLE and COMPOSING it with a WHITE CIRCLE? Can you accent a
>character by composing it with a spacing accent character?
>
> To avoid such problems, the COMPOSE character is only used in cases
>where the derivation of the character is clearly understood, and known
>to be overstuck. This is a historical judgement.
>
> It applies mostly to the A P L block, and there are 64 symbols,
>represented here by a sample of 2:
>
> APL FUNCTIONAL SYMBOL CIRCLE BACKSLASH = WHITE CIRCLE +
> PRESENTATION SUGGESTION COMPOSE + BACKSLASH
> APL FUNCTIONAL SYMBOL DOWN TACK UNDERBAR = DOWN TACK +
> PRESENTATION SUGGESTION COMPOSE + LOW LINE
>
> Composition could also be used to shrink the number of box-drawing
>characters down to a very reasonable 20 or so, from which the rest can
>be built.
>
> If widely deployed, the COMPOSE operation could cause no end of havoc
>by encouraging the creation of new symbols in a very uncontrolled way.
>(On the other hand, maybe that's a good thing.)
[snip]
>
> Jonathan Coxhead, 6 Jul 1999
There was such a character proposed several years ago for APL. I don't know
the whole history, but it was thrown out in the end. One of several
arguments against it was that some APL characters were unified with ASCII
or Math Symbol characters, but the complete mapping was never specified.
For that and other reasons it was entirely unclear which combinations could
actually be made.
There was also a triple overstrike defined at one time, representing the
sequence which was typed O^hU^hT, as it would appear on a printing
terminal. Since this was a signal to the APL system and could not appear in
an APL program, and wasn't in any APL character set, there was no reason
for it to be a Unicode character, even though it had to have a visual
representation in APL manuals and tutorials.
-- Edward Cherlin edward.cherlin.sy.67@aya.yale.edu "It isn't what you don't know that hurts you, it's what you know that ain't so."--Mark Twain, or else some other prominent 19th century humorist and wit
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