title: tensorflow scan on a matrix
body: <p>How to get the following example of scan to work?  I plan to use this example to test out some code in the function f.</p>
  
  <pre><code>def f(prev_y, curr_y):
    fval = tf.nn.softmax(curr_y)
    return fval
  
  a = tf.constant([[.1, .25, .3, .2, .15],
                   [.07, .35, .27, .17, .14]])
  c = tf.scan(f, a, initializer=0)
  with tf.Session() as sess:
    print(sess.run(c))
  </code></pre>