# how to remove additional space around \underbrace and justify the text under it

How do I

1. remove the additional space around \underbrace
2. justify the text under it, if the latter is longer than the text above it?

To illustrate the issue:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\underbracewithoutspace}[2]{\mathrlap{\underbrace{\phantom{#1\strut}}_{#2}}#1}

\begin{document}

$$\underbrace{a+b}_{\text{a long blah blah blah}}+c$$

% using the technique of
$$\underbracewithoutspace{a+b}{\text{a long blah blah blah}}+c$$

\end{document}

The first line (plain \underbrace) tries to center the text under the brace, while the second version (taken from this question: Remove additional space around \underbrace{…}) puts the brace under the wrong symbol.

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