title: Breaking long equations in R documentation
body: <p>I am trying to write a long equation in the documentation of a function in R.
  How can I break it into multiple lines? I tried adding <code>\cr</code> into the <code>\deqn</code> environment but the check returned me the following error</p>
  
  <blockquote>
    <p>LaTeX errors found: ! Misplaced \cr.  \tilde {Var} = \cr 
                                  \sum _{i \in U} \pi <em>i \Bigl ( 1 - \frac {n-1}... l.298 ...{Y}{n}\sum</em>{i\in U} \pi_i^2 \Biggr)^2 }{}</p>
  </blockquote>
  
  <p>I am writing documentation using <code>roxygen2</code>, but I also tried to add <code>\cr</code> manually to the <code>Rd</code> file and got the same result.</p>
  
  <p>Here is an example of my code without <code>\cr</code></p>
  
  <pre><code>#' @details
  #'
  #' \itemize{
  #'     
  #' \item Equation:
  #'
  #'     \deqn{ \tilde{Var} =
  #'     \sum_{i \in U} \pi_i \Bigl( 1 - \frac{n-1}{n}\pi_i \Bigr) 
  #'     \Biggr( \frac{y_i}{\pi_i} - \frac{Y}{n}  \Biggr)^2
  #'     - \frac{n-1}{n^2} \sum_{i \in U} \Biggl( 2\pi_i^3 - \frac{\pi_i^2}{2}
  #'     \sum_{j \in U} \pi_j^2 \Biggr)\Biggr( \frac{y_i}{\pi_i} - 
  #'    \frac{Y}{n}  \Biggr)^2
  #'     + \frac{2(n-1)}{n^3} \Biggl( \sum_{i \in U}\pi_i y_i - \frac{Y}
  #'    {n}\sum_{i\in U} \pi_i^2 \Biggr)^2 }{}
  #'
  #'}
  </code></pre>
  
  <p>and with <code>\cr</code></p>
  
  <pre><code>#' @details
  #'
  #' \itemize{
  #'     
  #' \item Equation:
  #'
  #'     \deqn{ \tilde{Var} =
  #'     \sum_{i \in U} \pi_i \Bigl( 1 - \frac{n-1}{n}\pi_i \Bigr) 
  #'     \Biggr( \frac{y_i}{\pi_i} - \frac{Y}{n}  \Biggr)^2 \cr
  #'     - \frac{n-1}{n^2} \sum_{i \in U} \Biggl( 2\pi_i^3 - \frac{\pi_i^2}{2}
  #'     \sum_{j \in U} \pi_j^2 \Biggr)\Biggr( \frac{y_i}{\pi_i} - 
  #'    \frac{Y}{n}  \Biggr)^2 \cr
  #'     + \frac{2(n-1)}{n^3} \Biggl( \sum_{i \in U}\pi_i y_i - \frac{Y}
  #'    {n}\sum_{i\in U} \pi_i^2 \Biggr)^2 }{}
  #'
  #'}
  </code></pre>