\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(75, 31)(3,-20)
    \gasset{Nw=1,Nh=1,Nmr=0.5,Nframe=n,curvedepth=0}
    \thinlines
    \node(A0)(0,0){$\bullet$}
    \node(Aa)(15,10){$\bullet$}
    \node(Ab)(15,-10){$\bullet$}
    \node(Aba)(30,0){$\bullet$}
    \node(Abaa)(45,10){$\bullet$}
    \node(Abaab)(60,0){$\bullet$}
    \node(Abaaba)(75,10){$\bullet$}
    \node(Abb)(30,-20){$\bullet$}
    \node(Abba)(45,-10){$\bullet$}
    \put(15,12){$a$}
    \put(75,12){$baaba$}
    \put(45,-13){$bba$}
    \drawedge[AHnb=0,ELside=l](A0,Aa){}
    \drawedge[AHnb=0,ELside=l](A0,Ab){}
    \drawedge[AHnb=0,ELside=l](Ab,Aba){}
    \drawedge[AHnb=0,ELside=l](Aba,Abaa){}
    \drawedge[AHnb=0,ELside=l](Abaa,Abaab){}
    \drawedge[AHnb=0,ELside=l](Abaab,Abaaba){}
    \drawedge[AHnb=0,ELside=l](Ab,Abb){}
    \drawedge[AHnb=0,ELside=l](Abb,Abba){}
    \end{picture}
  \end{center}
  \caption{Le code préfixe $\{a,bba,baaba\}$.}
\end{figure}