[Tex/LaTex] Remove numbering from Proofs

environmentsnewtheoremtheorems

I have this MWE. How can I remove the numbering from the Proofs? Note that another alternative is to use amsthm package. However, in my case, it displays Command \openbox already defined. \vrule\hfil}}

 \documentclass[12pt]{article}  
 \usepackage{amsmath}  
 \usepackage{amssymb}   
 %\usepackage{amsthm} % when I use this, I remove the hyperref and cleveref packages
 %\newtheorem{theorem}{Theorem}
 %\newtheorem{lemma}{Lemma}
 %\newtheorem{corollary}{Corollary}
 %\newtheorem{remark}{Remark} 
 \usepackage{hyperref}  
 \usepackage{cleveref}
 \newtheorem{thm}{Theorem}
 \newtheorem{lemma}[thm]{Lemma}
 \newtheorem{corollary}{Corollary}
 \newtheorem{remark}{Remark}
 \newtheorem{proposition}{Proposition}
 \newtheorem{example}{Example}
 \newtheorem{proof}{Proof}
 \newtheorem{definition}{Definition} 

 \begin{document}  

  \begin{lemma}   
    My lemma  
    \end{lemma}  

 \begin{proof}
   My Proof
 \end{proof}

 \end{document}

Best Answer

Here's a possibility:

\documentclass[12pt]{article}  
\usepackage{hyperref}  
\usepackage{cleveref}
\usepackage{ntheorem}
\newtheorem{thm}{Theorem}
\newtheorem{lemma}[thm]{Lemma}
\newtheorem{corollary}{Corollary}
\newtheorem{remark}{Remark}
\newtheorem{proposition}{Proposition}
\newtheorem{example}{Example}
\newtheorem{definition}{Definition} 
\theoremstyle{nonumberplain}
\newtheorem{proof}{Proof}
\begin{document}  

\begin{lemma}   
  My lemma  
\end{lemma}  

\begin{proof}
  My Proof
\end{proof}

\end{document}
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