[luatex] Expansion of TeX macros

[Dirk Laurie]

I've written a Lua function which combines with a LaTeX
command so I can write
   \mat{1,2,3,4;5,6,7,8}
and get the same effect as
  \begin{bmatrix} 1 & 2 & 3 & 4 \\ 5 & 6 & 7 & 8 \end{bmatrix}

The Lua is perfecrtly straightforwad: couple of gsub's translating
the commas and semicolons.

Fine, but when the matrix has some hard TeX in it, e.g.

  \mat{\mat{X,\mathbf 0;\mathbf c',1}

the \mathbf gets expanded to an unbelievable string of macros,
containing, alas, stuff that confuses the Lua code.  I've tried
\noexpand and \expandafter, neither of which I understand
fully, to no avail.