Left-aligning multiline \txt nodes in xymatrix

Question

I can write something like this:

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\[
\xymatrix{
    2 \times 2 \ar@/_2pc/[ddd]_{\txt{generalize}} \\ \\ \\
    \txt{$\Lambda N.\, \lambda$ $(\text{MUL} : N \to N \to N)$ \\
        $(\text{O} : N)$ \\
        $(\text{S} : N \to N).\,$ \\
        MUL (S (S O)) (S (S O))} \ar@/_2pc/[uuu]_{\txt{specialize}}
}
\]
\end{document}

and it looks like this:

I would like to left-align the text; I want to end up with something like this:

which I could do by using \phantom if I could get \txt to be left-aligned here. How do I get left-alignment inside \txt in \xymatrix?

Answer 1

I would use a tabular; the trick is to use an additional pair of braces.

Instead of adding rows, it's better to act on the row spacing, as shown below.

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}

\[
\xymatrix@R+6pc{
  2 \times 2 \ar@/_2pc/[d]_{\txt{generalize}} \\
  \txt{{%
    \begin{tabular}[t]{@{}l@{ }l@{}}
    $\Lambda N.\, \lambda$ & $(\text{MUL} : N \to N \to N)$ \\
                           & $(\text{O} : N)$ \\
                           & $(\text{S} : N \to N).\,$ \\
    \multicolumn{2}{@{}c@{}}{MUL (S (S O)) (S (S O))}
    \end{tabular}%
  }} \ar@/_2pc/[u]_{\txt{specialize}}
}
\]

\end{document}