Typography of nested quotations

Question

In the case of nested quotations, we have the following rule in French typography: if two or more closing quotation marks follow one another, we will write only one closing quotation mark.

Thus:

« Bla bla « bla bla bla « bla bla »»»

will become

« Bla bla « bla bla bla « bla bla »

But

« Bla bla « bla « bla bla » bla » bla »

stay

« Bla bla « bla « bla bla » bla » bla »

How do you do that in LaTeX?

Here's where I'm stuck: I can't implement the "quotation marks must follow each other" condition.

\documentclass{article}
\usepackage[french]{babel}
\usepackage{ifthen}

\makeatletter

\newcounter{nesting@depth}

\providecommand{\guillemets}[1]{%
    \addtocounter{nesting@depth}{1}%
    \og%
    #1%
    \ifthenelse%
        {\value{nesting@depth}>1}%
        {}%
        {\fg{}}%
    \addtocounter{nesting@depth}{-1}%
}

\makeatother

\begin{document}

\guillemets{Bla bla \guillemets{ bla bla bla \guillemets{bla bla}}}

\end{document}

Answer 1

Use the csquotes package:

\documentclass{article}

\usepackage[french]{babel}
\usepackage[autostyle,maxlevel=3,french=guillemets]{csquotes}

\begin{document}

\enquote{Bla bla \enquote{bla bla bla \enquote{bla bla}}}

\end{document}