title: lag not working as expected
body: <p><code>lag</code> is not working as I expected</p>
  
  <pre><code>a&lt;-c(0,1,2,3,4,5,6,7,8)
  a
  ## [1] 0 1 2 3 4 5 6 7 8
  lag(a,k=1)
  ## [1] 0 1 2 3 4 5 6 7 8
  ## attr(,"tsp")
  ## [1] 0 8 1
  </code></pre>
  
  <p>I thought I would get:</p>
  
  <pre><code>0 0 1 2 3 4 5 6 7 
  </code></pre>
  
  <p>or</p>
  
  <pre><code>1 2 3 4 5 6 7 8 0
  </code></pre>
  
  <p>What am I doing wrong?</p>