\unprotect
\newtoks\toks@
\let\z@\zeropoint
\protect
\usemodule[tikz]

\starttext

\starttikzpicture[->,>=stealth',shorten >=1pt,auto,node distance=4cm,
                    semithick]
  \tikzstyle{state}=[fill=gray,draw=none,text=white]

  \node[state]         (s0) [label=left:$s_0$] {$p,q$};
  \node[state]         (s1) [below right of=s0,label=right:$s_1$] {$q$};
  \node[state]         (s2) [below of=s0,label=right:$s_2$] {$q$};
  \node[state]         (s3) [below left of=s0,label=left:$s_3$] {$p$};

  \path (s0) edge [loop above] (s0)
             edge              (s1)
             edge              (s3)
        (s1) edge [bend right] (s3)
        (s2) edge [loop below] (s2)
             edge              (s0)
             edge [right]      (s1)
        (s3) edge [right]      (s1)
             edge [right]      (s2);

\stoptikzpicture


\stoptext