title: Convert an upper to lower triangle matrix in r
body: <p>I looked inot several posts, but I still couldn't find a way to just transform an upper triangle matrix into a lower triangle matrix.</p>
  
  <p>I just want this: </p>
  
  <pre><code>    1   2   3
  a   10  11  12
  b       13  14
  c           15
  </code></pre>
  
  <p>To become this:</p>
  
  <pre><code>    a   b   c
  1   10
  2   11  13
  3   12  14  15
  </code></pre>