Unnumbered theorem-like environments using thmtools

Question

This produces an error (\* already defined):

\documentclass{report}
\usepackage{thmtools}
\declaretheorem[numbered=no]{theorem}
\begin{document}
     Nothing.
\end{document}

Adding package amsthm solves the problem:

\documentclass{report}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheorem[numbered=no]{theorem}
\begin{document}
     Nothing.
\end{document}

But what should I do if I would like to define my own proof environment (without changing my existing LaTeX code)? This produces an error (\proof already defined):

\documentclass{report}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheorem[numbered=no]{proof}
\begin{document}
     Nothing.
\end{document}

I could use package ntheorem as the backend of package thmtools, but the documentation of thmtools says that some features do not work with ntheorem.

Answer 1

You can simply fool LaTeX in thinking that proof is not defined:

\let\proof\relax
\declaretheorem[numbered=no]{proof}