Second, using UEB for it, we would transcribe a bidirectional arrow over AB as follows:
;;<,,ab>.9\wro
You were close. We do need a grade 1 word indicator to start. We do need braille grouping indicators around AB so that the modifier applies to both letters and not just to B. And, your bidirectional arrow is good. \wro
Because UEB does not classify “bidirectional arrow over” with the common modifiers in GTM §12, we should look to GTM §7.9 and use the indicator for “expression directly above.” .=.9
Then the arrow does not need any terminator or braille grouping indicators because its start and end are clearly delineated by the arrow indicator and an arrow terminator, respectively. .=| and .=o
Phew.
Braille on!
–Kyle