# Indent proof environment

This is standard proof environmet.

But I want to indent the body of proof like this. How should I code?

\documentclass[a4paper,12pt,oneside]{book}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{titlesec,fancyhdr}
\usepackage{amssymb,amsmath,amsthm,amsfonts,cleveref,enumitem,thmtools}
\usepackage[no-math]{fontspec}
\usepackage{xcolor}
\usepackage{setspace,mathtools}

• there is no "standard" proof environment, but many packages define one, you have tagged this proof-package are you using a package of that name? (I do not have that) or amsthm, or theorem or ntheorem or .... Nov 24 '15 at 11:12
• Please tell us how the proof environment is defined or, alternatively, how you generated the upper screenshot.
– Mico
Nov 24 '15 at 11:12

You can redefine the proof environment:

\documentclass{article}
\usepackage{amsthm}
\usepackage{lipsum} % just for the example

\makeatletter
\renewenvironment{proof}[1][\proofname]{\par
\pushQED{\qed}%
\normalfont \topsep6\p@\@plus6\p@\relax
\list{}{%
\settowidth{\leftmargin}{\itshape\proofname:\hskip\labelsep}%
\setlength{\labelwidth}{0pt}%
\setlength{\itemindent}{-\leftmargin}%
}%
}{%
\popQED\endlist\@endpefalse
}
\makeatother

\begin{document}

\lipsum[2]

\begin{proof}
\lipsum*[3]
\end{proof}

\end{document}