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%latex
\newcommand*{\mtrx}[1]{\begin{bmatrix}#1\end{bmatrix}}
$\mtrx{1 & 2 \\ 3 & 4 \\}$

%md(hide=0)
$\newcommand{\mtrx}[1]{\begin{bmatrix}#1\end{bmatrix}}
\mtrx{1 & 2 \\ 3 & 4 \\}$

[1234]\newcommand{\mtrx}[1]{\begin{bmatrix}#1\end{bmatrix}} \mtrx{1 & 2 \\ 3 & 4 \\}


%md(hide=0)
matrx is still defined:

$$\mtrx{\alpha & 2 \\ 3 & 4 \\}$$

matrx is still defined:

[α234]\mtrx{\alpha & 2 \\ 3 & 4 \\}