Superpose a subscript and the rest of a formula

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?

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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}

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I would suggest to try also \smashoperator[r]{...}; this depends on the "long text": if not very long, pushing it under the equals sign may be OK, but not if it goes beyond it. –  egreg Jul 28 '11 at 10:48