Align text using align* environment

Question

I would like to align text using the align* environment.

What I would like to do in the example below is to align the different elements inside the braces after # meaning. So, I would like to have indefinite(?x), masculine(?x) and singular(?x) below each other and aligned.

I have tried inserting another align* environment, but this gives me the error: Argument of \align* has an extra }

How can I do this? If this can be done in another way, i.e. not using the align* environment, this is fine for me either.

\documentclass{article}
\usepackage{amsmath}
\begin{document} 

\begin{footnotesize}
    \begin{align*}
    &\left[
        \begin{tabular}{l}
            \textbf{?un-word}\\
            \hline \hline
            args: \lbrack ?x \rbrack\\
            syn-cat: \\
            \qquad lex-cat: article \\
            \qquad number: singular \\
            \qquad gender: masculine \\
        \end{tabular}
    \right]
    \leftarrow
    &\left[
        \begin{tabular}{l}
            \textbf{?un-word}\\
            \hline \hline
            \# meaning: \{indefinite(?x), masculine(?x), singular(?x)\}\\
            \hline
            \# form: \{string(?un-word, un)\}\\
        \end{tabular}
    \right]
    \tag{un-cxn}
    \end{align*}
\end{footnotesize}
\end{document}

Answer 1

I think it's not possible to use align environments inside tabular, but you can use tabular inside other tabulars. In this case you can align all elements after meaning with a new tabular with option [t] to align its top line with previous text. The final brace } is included after last element, because I don't know how to align with bottom tabular line.

\documentclass{article}
\usepackage{amsmath}
\begin{document} 

\begin{footnotesize}
    \begin{align*}
    &\left[
        \begin{tabular}{l}
            \textbf{?un-word}\\
            \hline \hline
            args: \lbrack ?x \rbrack\\
            syn-cat: \\
            \qquad lex-cat: article \\
            \qquad number: singular \\
            \qquad gender: masculine \\
        \end{tabular}
    \right]
    \leftarrow
    &\left[
        \begin{tabular}{l}
            \textbf{?un-word}\\
            \hline \hline
            \# meaning: \{\begin{tabular}[t]{@{}l@{}}
                                    indefinite(?x),\\ 
                                    masculine(?x),\\
                                    singular(?x)\}
                                    \end{tabular}\\
            \hline
            \# form: \{string(?un-word, un)\}\\
        \end{tabular}
    \right]
    \tag{un-cxn}
    \end{align*}
\end{footnotesize}
\end{document}