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.

\newcommand{\undertab}[1]{\clap{%

• Thanks! What I'm after is not justification per se, it's correct positioning of the text/formula under the brace if it's longer than what's above it. As a more minor issue: is there a way of aligning what's above the brace with what's below it? In this example, I'm after aligning the a in the 'a long ' with the a in the formula. – Zhenya Sep 22 '11 at 19:26