[texhax] A Question about the Itemize Block

[Robert Sherry]

    First, I want to thank the group for their wonderful responses to my
questions
about bibliography. Now, please consider the following LaTeX file:

\documentclass[a4paper]{article}
\usepackage{pst-node}
\usepackage{amsthm}
\newtheorem{lemma}{Lemma}
\begin{document}
\begin{lemma}
All graphs are colorable
\end{lemma}
\begin{proof}
\par
We have two cases to consider:
\begin{itemize}
\item[Case 1:]
$v_{0}$ and $v_{1}$ were assigned colors in the same call to BFS
In this case, their distance from the root modulus 2 must be the same. In
addition, their distance from the root can differ by no more then one,
from the root. If they are at the same level from the root, then they would
\item[Case 2:]
$v_{0}$ and $v_{1}$ were assigned colors in different calls to BFS
\end{itemize}
\end{proof}
\end{document}

Normally, when I use LaTeX for doing proofs, it keeps the left hand margin
straight. In this document, the labels Case 1: and Case 2: are moved into
the left margin.
As a result, my output does not look so good. How can I prevent LaTeX from
doing this.

                              Robert Sherry