我在编译以下代码时遇到错误，因为我shorten>=7.5pt, shorten<=7.5pt在一个箭头命令中有一个可选键，shorten=7.5pt在另一个箭头命令中也有一个可选键。我收到的错误消息是“Package pgfkeys 错误：我不知道您向其传递了‘7.5pt’的键‘/tikz/shorten’。”我在其他文件中使用了这些键，它们都通过了编译。
\documentclass[10pt]{amsart}
\usepackage{mathtools,array}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc,positioning,intersections}
\usepackage{pgfplots}
\pgfplotsset{compat=1.11}
\setlength{\oddsidemargin}{0.0in}
\setlength{\evensidemargin}{0.0in} \setlength{\textwidth}{6.1in}
\setlength{\topmargin}{0.0in} \setlength{\textheight}{9in}
\makeatletter
\newcases{mycases}{\quad}{%
\hfil$\m@th\displaystyle{##}$}{$\m@th\displaystyle{##}$\hfil}{\lbrace}{.}
\makeatother
\begin{document}
\noindent \hspace*{\fill}
\begin{tikzpicture}[nodes={inner sep=0, font=\scriptsize}]
%Five levels of a tree are drawn, depicting various subintervals of the closed interval [0,1].
%Here is the first level.
\coordinate (vertex_for_interval_between_0_and_1) at (0,0);
\node[anchor=north] (southern_edge_of_node_for_vertex_for_interval_between_0_and_1) at ($(vertex_for_interval_between_0_and_1) +(0,-0.1)$){$[0,1]$};
\draw[fill] (0,0) circle (1.5pt);
\coordinate (tail_of_arrows_from_vertex_for_interval_between_0_and_1) at (southern_edge_of_node_for_vertex_for_interval_between_0_and_1);
%
%Here is the second level.
\draw[fill] (-0.75,-1.75) circle (1.5pt);
\draw[fill] (0.75,-1.75) circle (1.5pt);
\coordinate (vertex_for_interval_between_0_and_1/2) at (-0.75,-1.75);
\node[anchor=north] at ($(vertex_for_interval_between_0_and_1/2) +(0,-0.1)$){$[0,$\raisebox{0.2em}{$\scriptstyle{1}$}\hspace{-0.2em}/\hspace{-0.2em}\raisebox{-0.2em}{$\scriptstyle{2}$}$]$};
\coordinate (vertex_for_interval_between_1/2_and_1) at (0.75,-1.75);
\node[anchor=north] at ($(vertex_for_interval_between_1/2_and_1) +(0,-0.1)$){$[$\raisebox{0.2em}{$\scriptstyle{1}$}\hspace{-0.2em}/\hspace{-0.2em}\raisebox{-0.2em}{$\scriptstyle{2}$}$,1]$};
%
%
%Arrows are drawn between the diagrams.
\draw[-latex, line width=0.8pt, shorten>=7.5pt, shorten<=7.5pt] (southern_edge_of_node_for_vertex_for_interval_between_0_and_1.south) to node[midway, anchor=south, yshift=0.25mm, sloped]
{$\raisebox{0.2em}{$\scriptstyle{1}$}\hspace{-0.2em}/\hspace{-0.2em}\raisebox{-0.2em}{$\scriptstyle{2}$}$} (vertex_for_interval_between_0_and_1/2.north);
%\draw[-latex, line width=0.8pt, shorten=7.5pt] (southern_edge_of_node_for_vertex_for_interval_between_0_and_1.south) to node[midway, anchor=south, yshift=0.25mm, sloped]
%{\raisebox{0.2em}{$\scriptstyle{1}$}\hspace{-0.2em}/\hspace{-0.2em}\raisebox{-0.2em}{$\scriptstyle{2}$}} (vertex_for_interval_between_1/2_and_1.north);
\end{tikzpicture}
\hspace{\fill}
\end{document}