\documentclass{article}
\usepackage{gastex}

\begin{document}

% Nodes whose shape is a regular polygon

\begin{center}
\begin{picture}(120,30)(0,0)
%   \put(0,0){\framebox(120,30){}}
  \rpnode[polyangle=90,Nmarks=i,iangle=-90](A)(5,14)(3,5){A}
  \rpnode[arcradius=2,Nmarks=r](B)(35,17)(6,10){B}
  \nodelabel[ExtNL=y,NLangle=30,NLdist=0.5](B){B}
  \nodelabel[ExtNL=y,NLangle=60,NLdist=0.5](B){B}
  \rpnode[arcradius=2,polyangle=90](C)(70,12)(5,7){C}
  \imark[iangle=198](C)\fmark[fangle=18](C)
  \rpnode[Nmarks=fr,fangle=45](D)(105,15)(4,7){D}
  \drawloop(A){$a$}
  \drawloop[loopangle=90](C){$c$}
  \drawloop[loopangle=-45](D){$d$}
  \drawedge(A,B){$x$}
  \drawbpedge(B,-30,30,C,140,30){$y$}
  \drawqbpedge(C,37,D,83){$z$}
  \drawedge[curvedepth=14,ELside=r](D,A){}
\end{picture}
\end{center}

\end{document}