\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(20, 50)(0,-9)
    \gasset{Nw=4,Nh=4,Nmr=2,curvedepth=0}
    \thinlines
    \node[Nmarks=i,iangle=180](Aa)(0,0){$a$}
    \node[Nmarks=i,iangle=180](A1)(0,20){$1$}
    \node[Nmarks=i,iangle=0](Ao)(20,0){$o$}
    \node[Nmarks=i,iangle=0](A3)(20,20){$3$}
    \node[Nmarks=i,iangle=90](A4)(10,35){$4$}
    \drawedge[ELside=r](Aa,Ao){$a$}       \drawedge(Ao,Aa){}
    \drawedge(Aa,A1){$1, 3$}    \drawedge(A1,Aa){}
    \drawedge(A3,Ao){$1, 3$}    \drawedge(Ao,A3){}
    \drawedge(Aa,A3){}          \drawedge(A3,Aa){}
    \put(6.9,14){$1, 3$}
    \drawedge(Ao,A1){}          \drawedge(A1,Ao){}
    \put(6.9,4.5){$1, 3$}
    \drawedge(A1,A3){$a, o, 4$} \drawedge(A3,A1){}
    \drawedge(A1,A4){$3$}       \drawedge(A4,A1){}
    \drawedge(A4,A3){$3$}       \drawedge(A3,A4){}
    \drawloop[loopangle=180](A1){$a, o$}
    \drawloop[loopangle=270](Aa){$o, 4$}
    \drawloop[loopangle=270](Ao){$4$}
    \drawloop[loopangle=0](A3){$a, o$}
    \gasset{curvedepth=21}
    \drawedge(Aa,A4){$a$}
    \drawedge(A4,Ao){}
    \gasset{curvedepth=-21}
    \drawedge(A4,Aa){}
    \drawedge[ELside=r](Ao,A4){$o$}
    \end{picture}
  \end{center}
  \caption{Les chemins interdits.}
\end{figure}