I'm writing a little array whose elements are matrices of different sizes, and I want them aligned by their top rows.

Here's the sort of thing:

```
\documenstyle{article}
\use{mathtools}
\begin{document}
\begin{array}{ccc}
I_2&I_3&I_4\\
\hline
\begin{bmatrix}
   1&0\\ 0&1
\end{bmatrix}
\begin{bmatrix}
   1&0&0\\ 0&1&0\\ 0&0&1
\end{bmatrix}
\begin{bmatrix}
   1&0&0&0\\ 0&1&0&0\\ 0&0&1&0\\ 0&0&0&1
\end{bmatrix}
\end{array}
\end{document}

```

But the matrices are vertically centred:

[![LaTeX matrices](https://i.stack.imgur.com/5ZpRC.png)](https://i.stack.imgur.com/5ZpRC.png)

I can get the matrices vertically aligned by their top rows by using `raisebox` but this is fiddly:

```
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[
  \begin{array}{ccc}
    I_2&I_3&I_4\\
    \hline
    \rule{0mm}{11mm}\raisebox{4mm}{\begin{bmatrix}
      1&0\\ 0&1
    \end{bmatrix}}
    &
    \raisebox{2mm}{\begin{bmatrix}
      1&0&0\\ 0&1&0\\ 0&0&1
    \end{bmatrix}}
    &
    \begin{bmatrix}
      1&0&0&0\\ 0&1&0&0\\ 0&0&1&0\\ 0&0&0&1
    \end{bmatrix}
  \end{array}
\]
\end{document}

```

[![LaTeX matrices aligned](https://i.stack.imgur.com/FCpyr.png)](https://i.stack.imgur.com/FCpyr.png)

This is the effect I want - but surely there's a better way of obtaining this alignment than trial-and-error with `raisebox`?