How to increase the line spacing of the begin{proof} environment?

Question

I wish to change the spacing in amsthm's proof environment (in my case, increase the spacing). I have included my attempt. I do not want numbering so I included the *. However, my attempt fails because it does not include the QED symbol at the end of the proof on the far right (which I need). Anyone have any ideas on how to get the increased spacing AND a QED symbol? I've included the output of my code so everyone can see the difference between my myplain2 environment and the begin{proof} environments (my code has the spacing I want, but no QED symbol, and the begin{proof} environment does not have the spacing I want, but does have the QED symbol).
newtheoremstyle{myplain2}
 {-baselineskiptopsep}   % ABOVESPACE
  {topsep}   % BELOWSPACE
  {normalfontsetstretch{2}}  % BODYFONT
  {0pt}       % INDENT (empty value is the same as 0pt)
  {bfseries} % HEADFONT
  {.}         % HEADPUNCT
  {5pt plus 1pt minus 1pt} % HEADSPACE
  {}   
theoremstyle{myplain2}
newtheorem*{proof_new}{Proof}

begin{proof}
Oh hi Mark.
Oh hi Mark.
Oh hi Mark.
end{proof}

begin{proof_new}
Oh hi Mark.
Oh hi Mark.
Oh hi Mark.
end{proof_new}

Abuzar Ghafari · Answer

You can use the setspace package to set the spacing of the proof with the space environment.
As an example is given below:
documentclass[12pt,a4paper]{book} 
usepackage{setspace}

renewcommand{baselinestretch}{1}

begin{document}
    
    Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.
    
    begin{spacing}{2}
        Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing.
    end{spacing}

Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown pri.

end{document}

egreg · Answer

I'm not sure what would be the reason. Anyway…
documentclass{article}
usepackage{amsmath}
usepackage{amsthm}
usepackage{setspace}

usepackage{lipsum} % for mock text

% spaced proof
newenvironment{sproof}
 {parvspace{-topsep}begin{spacing}{1.5}begin{proof}}
 {end{proof}end{spacing}parvspace{-topsep}}

begin{document}

lipsum[1][1-5]

begin{proof}
lipsum[2][1-5]
end{proof}

lipsum[1][1-5]

begin{sproof}
lipsum[2][1-5]
end{sproof}

lipsum[1][1-5]

end{document}

How to increase the line spacing of the begin{proof} environment?

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