We load the moustache brackets from XITS Math because they are not available in Latin Modern Math, which is the default math font. Of course, the technique also works for text fonts (but not in this page).
\definefontfallback [xits-fallback] [file:xits-math.otf] [023B0,023B1]
\definefontsynonym [MathRoman] [modern] [fallbacks=xits-fallback]
Consider this increasing sequence of moustached primes:
\lmoustache 11 \rmoustache <
\lmoustache 29 \rmoustache <
\lmoustache 31 \rmoustache <
\lmoustache 37 \rmoustache <
\lmoustache 41 \rmoustache
Our main fonts doesn’t contain emojis, so we want to take them from a specialized font.
And since emojis are spread over several Unicode blocks, we just say “take every unknown glyph from that emoji font” (which doesn’t work for all symbols, of course).
\definefontsynonym [Serif] [Pagella] [features=default,fallbacks=emojifallback]