title: R rbind while preserving order or rows in each data frame
body: <p>I need to merge 2 data frames while preserving their order of appearance in each data frame : </p>
  
  <pre><code>x = data.frame(a=1:3, b=2:4, c=3:5)
  y = data.frame(a=c(10, 20, 30), b=c(20, 30, 40), c=c(30, 40, 50))
  </code></pre>
  
  <p>What I want is : </p>
  
  <pre><code>&gt; z 
  
  a   b   c
  1   2   3
  10  20  30
  2   3   4
  20  30  40
  3   4   5
  30  40  50
  </code></pre>
  
  <p>But what rbind does is it's adding the second dataframe under the first.</p>