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 \ifx\havemjoproofbycases\undefined
  12 \def\havemjoproofbycases{1}
  13
  14
  15 % Used below to define pcases. The ``loadonly'' parameter prevents
  16 % a very bad interaction with the beamer document class.
  17 \usepackage[loadonly]{enumitem}
  18
  19 % Needed to perform division in the definition of \singleblskip.
  20 \usepackage{calc}
  21
  22 % A \baselineskip without the \baselinestretch scaling factor. Even
  23 % though \baselinestretch defaults to 1.0, it doesn't really have that
  24 % value. Thus to avoid division by zero, we need to do the ``is this
  25 % thing empty?'' hack.
  26 %
  27 % If we use \baselineskip instead of \singleblskip in our list, things
  28 % get real ugly when the text is e.g. double-spaced.
  29 %
  30 \newlength{\singleblskip}
  31 \setlength{\singleblskip}{
  32   \baselineskip/\real{
  33       \if\relax\detokenize{\baselinestretch}\relax
  34         \baselinestretch%
  35       \else
  36         1%
  37       \fi}
  38 }
  39
  40 % Using the enumitem package, we define a new type of list, called
  41 % ``pcases'' (proof by cases). Each case has a label with an arabic
  42 % numeral (the case number), but also a \thiscase identifier. The
  43 % macro \thiscase is defined below by the \case command, and gives the
  44 % name or conditions or whatever that distinguish one case from
  45 % another.
  46 \newlist{pcases}{enumerate}{1}
  47 \setlist[pcases]{
  48   label=\textit{Case~\arabic*}:~\protect\thiscase\textit{.},
  49   ref=\arabic*,
  50   align=left,
  51   listparindent=\parindent,
  52   parsep=\parskip,
  53   itemsep=\singleblskip}
  54
  55 % The optional argument here gets stuffed into the \thiscase macro, to
  56 % be called by pcases when it creates this list item. The \hfill is a
  57 % hack intended to force the proof to start on a new line, rather than
  58 % right after the colon. A \newline where the \hfill is does not work,
  59 % so we consume the rest of the line instead.
  60 \newcommand{\case}[1][]{
  61   \def\thiscase{#1}%
  62   \item \hfill\par\vspace{\singleblskip}
  63 }
  64
  65
  66 \fi