我正在尝试使用 tikz 创建一个非常基本的 Petri 网，但一直出现标题中显示的错误。我使用的代码如下。
\usetikzlibrary{arrows,shapes,snakes,automata,backgrounds,petri,positioning}
\begin{tikzpicture}[node distance=2cm,>=stealth',bend angle=45,auto]
\tikzstyle{place}=[circle,thick,draw=black!75,minimum size=12mm]
\tikzstyle{transition}=[rectangle,thick,fill=black,minimum height=10mm,minimum width=2mm,inner ysep=2pt, inner xsep=2pt]
\begin{scope}
\node [place,tokens=2,label=left:condition 1] (p1) {};
\node [place,label=left:condition 2] (p2) [below of=p1] {};
\node [place,label=left:condition 3] (p3) [below of=p2] {};
\node [transition,label=right:machine function] (q3) [right of=p2] {}
edge [pre] node {1} (p1)
edge [pre] node {2} (p2)
edge [pre] node {3} (p3);
\end{scope}
\end{tikzpicture}
奇怪的是，在我之前的文档中，我使用下面的代码来表示另一个 Petri 网示例，其中包含标记，并且运行良好，但大约一页之后上述代码失败了。任何帮助修复此问题的人都将不胜感激。谢谢！
\usetikzlibrary{arrows,shapes,snakes,automata,backgrounds,petri,positioning}
\begin{tikzpicture}[node distance=2cm,>=stealth',bend angle=45,auto]
\tikzstyle{place}=[circle,thick,draw=black!75,minimum size=12mm]
\tikzstyle{transition}=[rectangle,thick,fill=black,minimum height=10mm,minimum width=2mm,inner ysep=2pt, inner xsep=2pt]
\begin{scope}
\node [place,tokens=1,label=above:$p_1$] (p1) {};
\node [transition, label=above:$q_1$] (q1) [right of=p1] {}
edge [pre] (p1);
\node [place,label=above:$p_2$] (p2) [right of=q1] {}
edge [pre] (q1);
\node [transition,label=above:$q_2$] (q2) [below of=q1] {}
edge [pre] (p1)
edge [post] (p2);
\node [place,tokens=3,label=above:$p_3$] (p3) [below of=p2] {}
edge [pre] (q2);
\node [transition,label=above:$q_3$] (q3) [below right of=p2] {}
edge [pre] (p2)
edge [pre] (p3);
\node [place,tokens=2,label=above:$p_4$] (p4) [right of=q3] {}
edge [pre] (q3);
\end{scope}
\end{tikzpicture}