Long limits overhang and cause a symbol to become too wide

Question

Sometimes I need to make a union with a long subscript specifier at the bottom like so:

\documentclass{article}
\begin{document}
  \begin{equation*}
    \bigcup_{a \in \{\text{set that is very wide}\}} S_a
  \end{equation*}
\end{document}

The problem is that this produces:

How do I move the S_a over to the left?

Answer 1

You can use the \smashoperator command, from mathtools:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

  \begin{equation*}
    \smashoperator{\bigcup_{a \in \{\text{set that is very wide}\}}} S_a
  \end{equation*}

\end{document}