[O] Feature request: Add support for org-toggle-pretty-entities preview of org-entities-user which are more than one unicode character

[Ilja Kocken]

Hi there!

I've recently added some custom entries to my org-entities-user 
(below), but the ones that are more than one UTF-8 character long 
(most) do not show in my org-buffer with 
org-toggle-pretty-entities on. Is there a particular reason that 
I'm missing for why it will only preview single-character 
entities?

(setq org-entities-user
     '(("dO" "\\(\\delta^{18}\\)O" nil "&delta;<sup>18</sup>O" 
     "δ18O" "δ18O" "δ¹⁸O")
       ("eO" "\\({}^{18}\\)O" nil "<sup>18</sup>O" "18O" "18O" 
       "¹⁸O")
       ("dC" "\\(\\delta^{13}\\)C" nil "&delta;<sup>13</sup>O" 
       "δ13C" "δ¹³C" "δ¹³C")
       ("tC" "\\({}^{13}\\)C" nil "<sup>13</sup>C" "13C" "¹³C" 
       "¹³O")
       ("COt" "CO\\(_{2}\\)" nil "CO<sub>2</sub>" "CO2" "CO₂" 
       "CO₂")
       ("D" "\\Delta_{47}" t "&Delta;<sub>47</sub>" "Δ47" "Δ47" 
       "Δ₄₇")
       ("celsius" "\\(^{\\circ}\\)C" nil "&deg;C" "°C" "°C" "℃")
       ;; unit space, for something like 15\us{}kg
       ("us" "\\," nil "&nbsp;" " " " " " ")
       ("appr" "\\sim" t "&tilde;" "~" "~" "~")
       ;; for gps-coordinates
       ("degree" "^{\\circ}" t "&deg;" "°" "°" "°")
       ("arcminute" "^{'}" t "&prime;" "'" "′" "′")
       ("arcsecond" "^{''}" t "&Prime;" "\"" "″" "″")
       ))

Or am I going about this the wrong way in this particular case? I 
could of course also write yasnippets to autocomplete the full 
org-source, but I would like more control over what the latex 
export looks like (since I have to format everything according to 
strict latex conventions for scientific journals, so for example 
the default CO_2 -> CO\(_{\text{2}}\) is wrong).

Kind regards,

Ilja