# What is a proper context to write such proof by case style?

I am writing some proofs in latex and currently I would like to follow the standard procedure to organize my proofs in the following way:

I know how to use the latex math mode, as well as some context like equation or align. However, I just cannot figure out how should I start each proof with a case aligned on the left...

Could anyone shed some lights in this problem? Thank you a lot!

• Do you want to number the cases? – Bernard Apr 30 '17 at 21:22
• @Bernard, no I don't need to number the case. – lllllllllllll Apr 30 '17 at 21:58
• I would use a description environment. – Bernard Apr 30 '17 at 22:03
• If you decide in favor of description have a look at enumitem. – TeXnician May 1 '17 at 7:00

Quick, simple solution; you can create a new kind of itemized list with enumitem. Here you have to manually insert \newlines though.

\documentclass{article}
\usepackage{mathtools,enumitem,newpxtext}

\DeclarePairedDelimiter{\dbracket}{[\![}{]\!]}

\newlist{caselist}{itemize}{1}
\setlist[caselist]{
label=\textsl{Case},
labelindent=0pt, %customize value for distance from left margin to "Case"
leftmargin=4em, %customize value for distance from left margin to item content
labelsep=*,
align=left
}

\begin{document}
\noindent Text before
\begin{caselist}
\item {$$f_{Z} \dbracket{x \coloneqq 0}(\sigma_{i}) = [x \mapsto Z]\sigma_{i}$$}:\newline{}
Assume $$c_{i} = E, n$$ and $$\alpha(E) = \sigma_{i}$$\newline{}
Thus $$\sigma_{i+1} = f_{Z} [\![x \coloneqq 0]\!](\sigma_{i}) = [x \mapsto Z] \alpha(E)$$\newline{}
etc.
\item $$f_{Z} \dbracket{x \coloneqq 0}(\sigma_{i}) = [x \mapsto N]\sigma_{i}$$ where $$m \ne 0$$:\newline{}
Assume $$c_{i} = E, n$$ and $$\alpha(E) = \sigma_{i}$$\newline{}
etc.
\end{caselist}
\noindent Text after
\end{document}