title: "with" function behaviour
body: <p>Can anyone explain what causes the error in the last line of the code? Is it a bug?</p>
  
  <pre><code>&gt; ll &lt;- list(a=1, b=2)
  &gt; ee &lt;- as.environment(ll)
  &gt; ee
  &lt;environment: 0x0000000004d35810&gt;
  &gt; ls(ee)
  [1] "a" "b"
  &gt; with(ee, a)
  [1] 1
  &gt; with(ee, a - b)
  Error in eval(expr, envir, enclos) : could not find function "-"
  &gt; 
  </code></pre>