mjo-proof_by_cases.tex

   1 %
   2 % A proof-by-cases environment. This gives you a nicely-indented list
   3 % of cases for use in proofs.
   4 %
   5 % Example:
   6 %
   7 % Case 1 (x >= 0): herp.
   8 %
   9 % Case 2 (x < 0): derp.
  10 %
  11
  12 % Used below to define pcases.
  13 \usepackage{enumitem}
  14
  15 % Needed to perform division in the definition of \singleblskip.
  16 \usepackage{calc}
  17
  18 % A \baselineskip without the \baselinestretch scaling factor. Even
  19 % though \baselinestretch defaults to 1.0, it doesn't really have that
  20 % value. Thus to avoid division by zero, we need to do the ``is this
  21 % thing empty?'' hack.
  22 %
  23 % If we use \baselineskip instead of \singleblskip in our list, things
  24 % get real ugly when the text is e.g. double-spaced.
  25 %
  26 \newlength{\singleblskip}
  27 \setlength{\singleblskip}{
  28   \baselineskip / \real{
  29       \if\relax\detokenize{\baselinestretch}\relax
  30         \baselinestretch%
  31       \else
  32         1%
  33       \fi}
  34 }
  35
  36 % Using the enumitem package, we define a new type of list, called
  37 % ``pcases'' (proof by cases). Each case has a label with an arabic
  38 % numeral (the case number), but also a \thiscase identifier. The
  39 % macro \thiscase is defined below by the \case command, and gives the
  40 % name or conditions or whatever that distinguish one case from
  41 % another.
  42 \newlist{pcases}{enumerate}{1}
  43 \setlist[pcases]{
  44   label=\textbf{Case~\arabic*}:~\protect\thiscase\textbf{.},
  45   ref=\arabic*,
  46   align=left,
  47   leftmargin=0pt,
  48   listparindent=\parindent,
  49   parsep=\parskip,
  50   itemsep=\singleblskip}
  51
  52 % The optional argument here gets stuffed into the \thiscase macro, to
  53 % be called by pcases when it creates this list item. The \hfill is a
  54 % hack intended to force the proof to start on a new line, rather than
  55 % right after the colon. A \newline where the \hfill is does not work,
  56 % so we consume the rest of the line instead.
  57 \newcommand{\case}[1][]{
  58   \def\thiscase{#1}%
  59   \item \hfill\par\vspace{\singleblskip}
  60 }