if drawing.mode == 'manhattan' then
        drawing.mode = 'line'
      elseif drawing.mode == 'rectangle' or drawing.mode == 'square' then
        drawing.mode = 'polygon'
      end

* Insufficient handling of constraints when moving points. For example, if you
  draw a manhattan line and then move one of the points, you may not be able
  to hover on it anymore.

  There's two broad ways to fix this. The first is to relax constraints,
  switch the manhattan line to not be manhattan. The second is to try to
  maintain constraints. Either constrain the point to only move along one line
  (but what if it's connected to two manhattan lines?!), or constrain the
  other end of the line to move alongside. I'm not sure yet which would be
  more useful. Getting into constraints would also make the program more
  complex.
  Bottomline: at the moment moving points connected to manhattan lines,
  rectangles or squares can break drawings in subtle ways.