\begin{figure}[H]
  \begin{center}
  \unitlength=4pt
  \begin{picture}(40, 32)(0,-4)
  \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=0}
  \thinlines
  \node[Nmarks=i,iangle=180](A1)(20,20){$1$}
  \node(A2)(40,20){$2$}
  \node[Nmarks=f,fangle=270](A3)(40,0){$3$}
  \node[Nmarks=i,iangle=270](A6)(0,0){$6$}
  \node[Nmarks=f,fangle=0](A7)(20,0){$7$}
  \drawedge(A1,A2){$a$}
  \drawedge(A3,A1){$b$}
  \drawedge(A6,A7){$a,b$}
  \drawloop[loopangle=90](A1){$b$}
  \gasset{curvedepth=4}
  \drawedge(A2,A3){$b$}
  \drawedge(A3,A2){$a$}
  \end{picture}
  \end{center}
  \caption{Après émondage.}
\end{figure}