title: Inserting multiple rows into matrix, shifting existing rows
body: <h2>Question:</h2>
  
  <p>I'm looking for the most efficient way to insert multiple rows <code>R</code> into a matrix <code>M</code> before specified rows <code>I</code>, while shifting existing rows down.</p>
  
  <h2>Example:</h2>
  
  <pre><code>M = [1 1 1 1;
       2 2 2 2;
       3 3 3 3;
       4 4 4 4;
       5 5 5 5];
  I = [1 3 3 5];
  R = [-6 -6 -6 -6;
       -7 -7 -7 -7;
       -8 -8 -8 -8
       -9 -9 -9 -9];
  </code></pre>
  
  <p>The result should be the matrix:</p>
  
  <pre><code>[-6 -6 -6 -6
    1  1  1  1
    2  2  2  2
   -7 -7 -7 -7
   -8 -8 -8 -8
    3  3  3  3
    4  4  4  4
   -9 -9 -9 -9
    5  5  5  5]
  </code></pre>