(\ffd{1}{d\:r} \vs \:B) d\:A + (\ffd{1}{d\:r} \vs \:A) d\:B
/home/limg/www/LimgMath/eq! Undefined control sequence. ...le \mathstrut { (\ffd {1}{d\:r} \vs \:B) d\:A + (\ffd {1}{d\:r... l.34 $}