title: Why can't one have several `value.var` in `dcast`?
body: <p>Why can't one have multiple variables passed to <code>value.var</code> in <code>dcast</code>? From <code>?dcast</code>: </p>
  
  <blockquote>
    <p>value.var name of column which stores values, 
    see guess_value for default strategies to figure this out.</p>
  </blockquote>
  
  <p>It doesn't explicitly indicate that only one single variable can be passed on as value. If however I try that, then I get an error: </p>
  
  <pre><code>&gt; library("reshape2")
  &gt; library("MASS")
  &gt; 
  &gt; dcast(Cars93, AirBags ~ DriveTrain, mean, value.var=c("Price", "Weight"))
  Error in .subset2(x, i, exact = exact) : subscript out of bounds
  In addition: Warning message:
  In if (!(value.var %in% names(data))) { :
    the condition has length &gt; 1 and only the first element will be used
  </code></pre>
  
  <p>So is there a good reason for imposing this limitation? And is it possible to work around this (perhaps using <code>reshape</code>, etc.)? </p>