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. The ``loadonly'' parameter prevents
  13 % a very bad interaction with the beamer document class.
  14 \usepackage[loadonly]{enumitem}
  15
  16 % Needed to perform division in the definition of \singleblskip.
  17 \usepackage{calc}
  18
  19 % A \baselineskip without the \baselinestretch scaling factor. Even
  20 % though \baselinestretch defaults to 1.0, it doesn't really have that
  21 % value. Thus to avoid division by zero, we need to do the ``is this
  22 % thing empty?'' hack.
  23 %
  24 % If we use \baselineskip instead of \singleblskip in our list, things
  25 % get real ugly when the text is e.g. double-spaced.
  26 %
  27 \newlength{\singleblskip}
  28 \setlength{\singleblskip}{
  29   \baselineskip/\real{
  30       \if\relax\detokenize{\baselinestretch}\relax
  31         \baselinestretch%
  32       \else
  33         1%
  34       \fi}
  35 }
  36
  37 % Using the enumitem package, we define a new type of list, called
  38 % ``pcases'' (proof by cases). Each case has a label with an arabic
  39 % numeral (the case number), but also a \thiscase identifier. The
  40 % macro \thiscase is defined below by the \case command, and gives the
  41 % name or conditions or whatever that distinguish one case from
  42 % another.
  43 \newlist{pcases}{enumerate}{1}
  44 \setlist[pcases]{
  45   label=\textbf{Case~\arabic*}:~\protect\thiscase\textbf{.},
  46   ref=\arabic*,
  47   align=left,
  48   leftmargin=0pt,
  49   listparindent=\parindent,
  50   parsep=\parskip,
  51   itemsep=\singleblskip}
  52
  53 % The optional argument here gets stuffed into the \thiscase macro, to
  54 % be called by pcases when it creates this list item. The \hfill is a
  55 % hack intended to force the proof to start on a new line, rather than
  56 % right after the colon. A \newline where the \hfill is does not work,
  57 % so we consume the rest of the line instead.
  58 \newcommand{\case}[1][]{
  59   \def\thiscase{#1}%
  60   \item \hfill\par\vspace{\singleblskip}
  61 }