\documentclass{article}
\usepackage[usenames]{color}
\usepackage{gastex}

\begin{document}

% Petri Net

\begin{center}
\begin{picture}(50,50)(-25,-24)
\put(-25,-24){\framebox(50,50){}}
  \node(A)(-20,-7){$\bullet \bullet$}
  \node(B)(-20, 7){$\bullet$}
  \node(C)( 20,-7){}
  \node(D)( 20, 7){}
  \gasset{Nw=.7,Nh=7,Nmr=0,fillgray=0} % black rectangle
  \gasset{ExtNL=y,NLdist=1,NLangle=90} % external label above the node
  \node(Ta)(0,  0){$a$}
  \node(Tb)(0,-20){$b$}
  \node(Tc)(0, 20){$c$}
 
  \gasset{ELdistC=y,ELdist=0}
  \drawedge[ELside=r,eyo=-1](A,Ta){\colorbox{white}{2}}\drawedge[eyo=1](B,Ta){}
  \drawedge[syo=-1](Ta,C){}\drawedge[syo=1](Ta,D){}
  {\gasset{curvedepth=4}\drawedge(C,Tb){}\drawedge(Tb,A){\colorbox{white}{2}}}
  {\gasset{curvedepth=-4}\drawedge(D,Tc){}\drawedge[syo=1](Tc,B){}}
  \drawbpedge[syo=-1,ELpos=70](Tc,-160,15,D,170,20){\colorbox{white}{3}}
\end{picture}
\end{center}

\end{document}