Superpose a subscript and the rest of a formula

Question

I have a formula in the equation environment that goes like

Foo = \bigcup_{x \in veryveryveryverylongdescription} Bar(x)

Because the state space's description is long, the Bar(x) is rejected far away from the \bigcup. I would like Bar(x) to be right next to the union symbol, above the description.

Can this be done cleanly? Or shall I try to have with two equations, some vspace and horizontal space shifts?

Answer 1

Use the \smashoperator macro from the mathtools package:

\documentclass{article}

\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}

\[ \mathit{Foo} =
   \smashoperator{\bigcup_{x \in \text{long text}}} \operatorname{Bar}(x) \]

\end{document}