LyX doesn't handle nested braces correctly. In a maths inset write {
{1234 } } so that the outer pair of braces contains a single argument.
Now save and close the document, then reopen it and the maths inset will
contain { 1234 } i.e. 4 tokens rather than the single argument grouping
before. Depending on the surrounding commands, the results can be very
different. Adding an empty brace pair prevents this: the outer braces
are not stripped from { {1234} {} }, but one shouldn't have to.
I've written a package producing differential coefficients. The
denominator of a partial derivative is formed from a grouping like
{xyz} which produces \partial x\partial y\partial z. But in many
subjects variables are sub- or superscripted (e.g. coordinates x_1, x_2,
x_3 for x, y, z). In order that x_1 produce \partial x_1 it's necessary
to put it in double braces: { {x_1} }. If the outer braces are stripped,
as LyX does, latex ends up trying to form a 3-variable mixed partial
derivative out of x, _, and 1. (\partial x_\partial\partial 1 is what
eventually emerges past the latex errors.) LyX is creating the error by
stripping the outer braces when the document is reopened.
Andrew
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