title: How to limit UITableView row reordering to a section
body: <p>I was hitting my head over this one, and google was turning up nothing.
  I eventually worked it out and thought I'd write it up here for the sake of the next person.</p>
  
  <p>You have a <code>UITableView</code> with multiple sections. Each section is homogeneous, but the table overall is heterogeneous. So you might want to allow re-ordering of rows within a section, but not <em>across</em> sections. Maybe you only even want want one section to be reorderable at all (that was my case).
  If you're looking, as I was, at the <code>UITableViewDataSourceDelegate</code> you won't find a notification for when it is about to let you move a row between sections. You get one when it starts moving a row (which is fine) and one when it's already moved it and you get a chance to sync with your internal stuff. Not helpful.</p>
  
  <p>So how can you prevent re-orders between sections?</p>
  
  <p>I'll post what I did as a separate answer, leaving it open for someone else to post an even better answer!</p>