title: How do I concatenate subarrays of a list into one list; ie create a flat list?
body: <p>Assume I have array <code> arr = [[1, 2, 3], [4, 5, 6]]</code></p>
  <p>How do I concatenate this array into one array, such as <code>arr = [1, 2, 3, 4, 5, 6]</code></p>
  <p>Thanks in advance!</p>