title: Keras SimpleRNN input shape and masking
body: <p>Newbie to Keras alert!!!</p>
  
  <p>I've got some questions related to Recurrent Layers in Keras (over theano)</p>
  
  <ol>
  <li>How is the input supposed to be formatted regarding timesteps (say for instance I want a layer that will have 3 timesteps 1 in the future 1 in the past and 1 current) I see some <a href="https://stackoverflow.com/questions/36136562/python-keras-simplernn-wrong-number-of-dimensions-on-model-fit" title="input for simpleRNN question">answers</a> and the API proposing padding and using the embedding layer or to shape the input using a time window (3 in this case) and in any case I can't make heads or tails of the API and SimpleRNN examples are scarce and don't seem to agree.</li>
  <li>How would the input time window formatting work with a masking layer?</li>
  <li>Some related answers propose performing masking with an embedding layer. What does masking have to do with embedding layers anyway, aren't embedding layers basically 1-hot word embeddings? (my application would use phonemes or characters as input)</li>
  </ol>