[XeTeX] additional beginL endL nodes in math

[Arthur Reutenauer]

> I do not understand.  If system A performed function Y until now, yet
> its designer/maintainer wishes it to perform function Y' henceforth

  Khaled's point is that XeTeX did not work correctly on the issue at
hand until he made the change we're discussing.  It's unfair to
characterise his change as a change in behaviour of the TeX engine, it
was a bugfix.

	Arthur