title: NumPy matrix plus column vector
body: <p>I am using <code>numpy.matrix</code>. If I add a <code>3x3</code> matrix with a <code>1x3</code>, or <code>3x1</code>, vector, I get a <code>3x3</code> matrix back. </p>
  
  <p>Should this not be <code>undefined</code>?
  And if not, what is the explanation to this?</p>
  
  <p>Example:</p>
  
  <pre><code>a = np.matrix('1 1 1; 1 1 1; 1 1 1')
  b = np.matrix('1 1 1')
  a + b #or a + np.transpose(b)
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>matrix([[2, 2, 2],
          [2, 2, 2],
          [2, 2, 2]])
  </code></pre>