Acronym package: Handling possessive case (apostrophe s)

Question

How do I handle the possessive form of an acronym using the acronym package?

Acronym definition:

\begin{acronym}
\acro{FBI}{Federal Bureau of Investigation}
\end{acronym}

Usage:

The \ac{FBI}'s plan. The \ac{FBI}'s plan.

Result:

The Federal Bureau of Investigation (FBI)'s plan. The FBI's plan.

The short form is right, but the long form is wrong.

Answer 1

Here's a method that defines the command \acposs which will expand to long's (short) on first use and short's on next use:

\documentclass{article}
\usepackage{acronym}
\makeatletter
\newcommand{\acposs}[1]{%
 \expandafter\ifx\csname AC@\AC@prefix#1\endcsname\AC@used
   \acs{#1}'s%
 \else
   \aclu{#1}'s (\acs{#1})%
 \fi
}
\makeatother
\begin{document}
\begin{acronym}
\acro{FBI}{Federal Bureau of Investigation}
\end{acronym}
The \acposs{FBI} plan. The \acposs{FBI} plan.
\end{document}

If you prefer long's (short's) then use the following definition instead:

\newcommand{\acposs}[1]{%
 \expandafter\ifx\csname AC@#1\endcsname\AC@used
   \acs{#1}'s%
 \else
   \aclu{#1}'s (\acs{#1}'s)%
 \fi
}