title: Unable to get stack view to fill it's super view
body: <p>I am unable to get a stack view to fill the view canvas in order for my widgets to be automatically resized to fill the entire screen...  i've seen this done on may tutorial/youtube video but for some strange reason it doesn't seem to work for me
  How I went about it:
  I have a view controller scene where I am trying to display three buttons layed out horizontally with three labels beneath them.  I dragged the buttons and labels onto the view canvas from the object library and then added each button and it's  label to a verticle stack view.  Then I added the three verticle stack views to a horizontal stack view and set the distribution of the horizontal stack view to fill equally.  I then control-dragged from the horizontal stack view to view in the view controller scene in storyboard and set the following constraints.  (I zero'ed all the constants set automatically by storyboard and then have the following 4 constraints)</p>
  
  <pre><code>trailingMargin = Stack View.trailing
  Stack View.leading = leadingMargin
  Stack View.top = Top Layout Guide.bottom
  Bottom Layout Guide.top = Stack View.bottom
  </code></pre>
  
  <p>When I run the app I don't see anything displayed on the simulator's iPhone screen and I get a message saying that one or more of the constraints could not be satisfied.  Any idea what I am doing wrong?</p>