title: Programmatically creating "spacing to nearest neighbor" constraint
body: <p>Part of the screen I'm building includes a section with <code>n</code> views. I'm generating these views on the fly in code--they're simple <code>UIView</code> subclasses.</p>
  
  <p>I'm using AutoLayout constraints for this screen, and I'd like each view to automatically position itself 15px or so below the view above it.</p>
  
  <p>In Xcode it's possible to create a <a href="https://stackoverflow.com/questions/18959118/add-spacing-constraint-to-superview-in-xcode-5">spacing to nearest neighbor constraint</a>, which seems to do exactly what I want.</p>
  
  <p>However, I can't seem to find any examples that show how to create this in code.</p>
  
  <p>Is it possible to create a "spacing to nearest neighbor" constraint programmatically?</p>