title: Column and Row to row key
body: <pre><code>    [A] [B] [C]
  [A]  1   2   3
  [B]  4   5   6
  [C]  7   8   9
  </code></pre>
  <p>I have a matrix like above.</p>
  <p>I need to change it to</p>
  <pre><code>[row] [col] [value]
    A     A      1
    A     B      2
    A     C      3
    B     A      4
    B     B      5
         ...
  </code></pre>
  <p>So the original row names become the first key and original column names become the second key together forming a key. Like (row, col): value structure.</p>
  <p>How could I achieve such result?</p>