初回とそれ以降で評価される内容が異なるコマンドをつくる

MyDefinition.tex

\documentclass[a4paper]{jsarticle}
\usepackage{xspace}
\usepackage{etoolbox,etextools}

\makeatletter                                                                   
\newcommand*{\MyDefinition}[3]{%                                                
  \newtoggle{\string#1}%                                                        
  \togglefalse{\string#1}%                                                      
  \DeclareRobustCommand*#1{%                                                    
    \@ifstar{\csname s\string#1\endcsname}{\csname n\string#1\endcsname}}%      
  \@namedef{s\string#1}{%                                                       
    {#2\xspace}}%                                                               
  \@namedef{n\string#1}{%                                                       
    {\iftoggle{\string#1}{#3}{\global\toggletrue{\string#1}#2}}\xspace}}        
\makeatother                                                                    

\MyDefinition{\psf}{Point Spread Function (PSF)}{PSF}                           
\MyDefinition{\pfs}{Prime Focus Spectrograph (PFS)}{PFS}                        
                                                                                
\begin{document}                                                                
\section{\psf}                                                                  
{\psf} \psf* \psf \psf \psf*                                                    
                                                                                
\subsection{\pfs}                                                               
{\pfs} \pfs* \pfs \pfs \pfs*                                                    
\end{document}