The \lightning symbol is text mode only. Hence, for using it in
mathed, it must be inserted in a text mode environment such as
\mbox, \text, etc. However, as it is not marked as textmode,
LyX "helpfully" encloses it in \ensuremath everytime one tries
to put it in a text mode environment, thus defying the countermeasure.
By declaring it textmode only, LyX stops doing that and thus it can be
correctly output by simply selecting it and hitting Ctrl-M.
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