\begin{figure}[H]
  \begin{center}
     \unitlength=4pt
     \begin{picture}(20, 26)(0,-3)
     \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
     \thinlines
     \node(A0)(0,20){$0$}
     \node(A1)(20,20){$1$}
     \node(A2)(20,0){$2$}
     \node(A3)(0,0){$3$}
     \drawedge(A0,A1){$a$}
     \drawedge(A1,A2){$a$}
     \drawedge(A2,A3){$a$}
     \drawedge(A3,A0){$a$}
     \drawloop[loopangle=180](A0){$b$}
     \drawloop[loopangle=0](A1){$b$}
     \drawloop[loopangle=0](A2){$b$}
     \drawedge[curvedepth=-4,ELside=r](A3,A0){$b$}
     \end{picture}
   \end{center}
   \caption{L'automate de {\v C}ern{\'y}.}
\end{figure}