title: Why are = and <- not equivalent in within()?
body: <pre><code>&gt; within( list(a="a",b="b"), c="c" )
  Error in eval(expr, envir, enclos) : argument is missing, with no default
  &gt; within( list(a="a",b="b"), c&lt;-"c" )
  $a
  [1] "a"
  
  $b
  [1] "b"
  
  $c
  [1] "c"
  </code></pre>
  
  <p>I'm not sure exactly why these two shouldn't be equivalent.  It seems like the the <code>=</code> version getting interpreted as an argument named <code>c</code> to within because of the <code>...</code>.  Is there any way to disable this behavior?  I tried,</p>
  
  <pre><code>within( list(a="a",b="b"), `c`="c" )
  </code></pre>
  
  <p>but that fails too.</p>