Famous/physics — pull request #2 by malandrew (2014-04-11 19:20:47+00:00)
Title: Walls.DEFAULT_OPTIONS.sides fix

`Walls.TWO_DIMENSIONAL` -> `Walls.SIDES.TWO_DIMENSIONAL`

Also, FWIW, we might want to define the Walls.SIDES object on L126 before the Walls.DEFAULT_OPTIONS object defined on L49.