Very short overbrace

Question

Is there a good-looking way to produce an overbrace for a single letter? The overbrace produced by

\[
\overbrace{P}^{some annotation}
\]

is too wide for the single letter P. Note that the focus really lies upon the annotation, the reader has already seen the unannotated term and now gets to see an example which formal annotations are inserted where in the term.

Here is a simplified example from my sources. The overbraced P does not look good.

\documentclass{minimal}

\usepackage{proof}
\usepackage{mathtools}

\newcommand{\hoare}[3]{\{#1\}\;#2\;\{#3\}}
\newcommand{\ctxt}[3]{C:\langle#1,#2,#3\rangle}

\begin{document}

\begin{equation*}
\infer[Rule]
    {\underbrace{\hoare P c Q}_{\ctxt{i}{x \cdot \mathrm{xs}}{o_2}}}
    { \overbrace{\hoare{P'}{c'}{Q'}}^{\ctxt{i+1}{\mathrm{xs}}{o_1}}
    & R \implies \overbrace{P}^{\mathclap{\ctxt{o_1}{\text{a bit longer}}{o_2}}}
    }
\end{equation*}

\end{document}

Answer 1

In the end, I used the abraces package. This package provides commands to construct your own variants of overbrace/underbrace. I ended up using just the "middle" part of the overbrace, without the small arcs at the left and right end. This can be obtained by:

\aoverbrace[U]{formula}^{brace-formula}

Where necessary, I also used \mathclap from mathtools so that the annotation does not introduce to much whitespace into the rest of the formula.

\newcommand{\shortoverbrace}[2]{\aoverbrace[U]{#1}^{\mathclap{#2}}}%