\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(40,34)(20,15)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
    \thinlines
    \node[Nmarks=i,iangle=180](A1)(20,40){$1$}
    \node(A2)(40,40){$12$}
    \node[Nmarks=f,fangle=270](A3)(40,20){$13$}
    \node[Nmarks=f,fangle=270](A8)(60,20){$123$}
    \drawedge(A1,A2){$a$}
    \drawedge(A3,A1){$b$}
    \drawedge(A2,A8){$a$}
    \drawedge(A8,A3){$b$}
    \drawloop[loopangle=90](A1){$b$}
    \drawloop[loopangle=0](A8){$a$}
    \gasset{curvedepth=4}
    \drawedge(A2,A3){$b$}
    \drawedge(A3,A2){$a$}
    \end{picture}
  \end{center}
  \caption{...et émondage.}
\end{figure}