Using “csup” to superpose ≈ and + yields a pretty ugly result. The reason for this is that the plus sign is way too high. Using matrices is even worse because, in addition to the plus sign being too high, the ≈ sign is too low. I’m looking for something much more vertically compact, perhaps similar in layout to a symbol with an accent on top.
Thank you in advance for any help, and God bless you.
(Although I don’t know how to make it work…)
See unicode 1AC8 “COMBINING PLUS SIGN ABOVE, used in extended IPA” (https://www.unicode.org/charts/PDF/U1AB0.pdf).
You told us what you tried and gave an unsatisfactory result. Good. Now tell us which operator or known math symbol you want to insert. We might suggest a working solution. And, as usual, mention, OS name, LO version and save format.
If this is no math symbol but a tweak to get a special glyph in a Writer document, describe in layman words the effect you want to achieve.
Easy. In all LO components, you type the base character followed by the combining mark(s). In case the combining mark is not on your keyboard, type U+1ac8 followed by Alt+x. If there is no ambiguity, you can drop + or even U. (and, again, the trick is valid in all components.)
Thank you for your response.
It’s not a widely known symbol, as far as I know. I want a plus sign over a “almost-equal-to” sign to signify “was-rounded-upward-and-not-to-the-nearest-number”. Some people put a dot/bullet over an equal sign to signify “is-therefore/finally-equal-to”. I want to achieve a similar result, but with the too symbols I mentioned.
I’m running the Flatpak version of the LibreOffice suite on Debian GNU/Linux 11 (bullseye), and inserting equations into LO Writer via the integrated LO Math interface. I’m saving to ODT.
The Unicode trick sounded really cool, but doesn’t work as of now. I get an empty character, meaning that it is probably not supported by the font I’m using (Liberation Serif, I believe). I’m not sure I feel like testing every font on my system and see if it works with Unicode character 1AC8 but, if someone knows of a font that does support it, I’ll use that, I guess. (That is, once I figure out how to change an equation’s font. 
)
Thank you again.
What exactly did you try? (The code tokens to produce the “way too high” + sign.)
I would try one of these first:
a approx csup "+" B
… or …
a approx csup {{}_"+"} B
Is that what you have?
I know that (anyway, it is good to estate this here for who don’t know), but all I get is an empty square. I tested with all installed fonts, including Noto Sans/Serif.
As I explain in my answer, U+1AC8 resides in a “niche” block. The audience is an elite group of linguists. I guess this is very narrow market for font designers/providers. As a consequence, all fonts installed on my computer have no glyph in this block. This is why I looked for a solution based on the standard plus sign.
csup puts the centered subscript indeed way too high. Unfortunately there is no way in Format>Spacing to tune the distance between the base symbol and the subscript.
Using combining IPA marks stumps on the fact that these characters are targeted at a very specialised domain with a narrow audience. Consequently, the glyphs are not present in common fonts.
In the end I found a solution with the standard plus sign. The idea is to use a limit. You can suffix some operator with to { } or from { }. The remaining problem is to turn the “almost-equal-to” sign into an operator. I ended up with:
oper ≈ to{"+"}
Though the plus sign is still relatively far from the equal sign, it is better than with csup. And you can control the distance in Format>Spacing (only to increase it since it is at 0% initially).
This is quite satisfactory. As you’ve mentioned, it is much better than using csup. It’ll do!
God bless you.