\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(33, 41)(0,-8)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \gasset{curvedepth=0,Nw=5,Nh=5}
    \node[Nmarks=if,iangle=180,fangle=270](A1)(0,12.5){$1$}
    \node(A2)(12.5,25){$2$}
    \node[Nmarks=f,fangle=270](A3)(25,12.5){$3$}
    \node(A4)(12.5,0){$4$}
    \drawedge(A1,A2){$a$}
    \drawedge(A1,A4){$b$}
    \drawloop[loopangle=90](A2){$b$}
    \drawedge[curvedepth=4](A2,A3){$a$}
    \drawedge[curvedepth=4](A3,A2){$a$}
    \drawloop[loopangle=0](A3){$b$}
    \drawedge(A3,A4){$b$}
    \drawedge(A4,A2){$a$}
    \drawloop[loopangle=270](A4){$b$}
  \end{picture}
  \end{center}
  \caption{Un automate non déterministe.}
\end{figure}