\documentclass[12pt]{report} 
    \usepackage{amsmath} 
\begin{document} 
 
 
\begin{align} 
a & = a \\ 
f & = 
\begin{cases} 
a \\ 
b 
\end{cases} 
\end{align} 
 
\newenvironment{foo}{}{} 
\newenvironment{textequation}  {$$\begin{foo}}   {\end{foo}$$} 
 
   \Picture*{} \begin{textequation} a \end{textequation} \EndPicture 
 
   \begin{textequation} a \end{textequation} 
 
\renewenvironment{textequation}  {$$} {$$} 
 
   \Picture*{} \begin{textequation} a \end{textequation} \EndPicture 
 
   \begin{textequation} a \end{textequation} 
 
\renewenvironment{textequation}  {\begin{foo}\begin{foo}}   {\end{foo}\end{foo}} 
 
   \Picture*{} \begin{textequation} a \end{textequation} \EndPicture 
 
   \begin{textequation} a \end{textequation} 
 
\renewenvironment{textequation}  {\begin{foo}}   {\end{foo}} 
 
   \Picture*{} \begin{textequation} a \end{textequation} \EndPicture 
 
   \begin{textequation} a \end{textequation} 
 
\ConfigureEnv{textequation}   {\Picture*{}}   {\EndPicture}   {}{} 
 
   \Picture*{} \begin{textequation} a \end{textequation} \EndPicture 
 
   \begin{textequation} a \end{textequation} 
 
    xxxxxxx 
   \begin{equation} 
      g(E) = 
   \end{equation} 
   yyyyyyyy 
 
\end{document}