== What shouldn't be a method function

I dislike excessive religion, so I don't hold to the view that
everything should be a method function. Fortunately, Python lets me
indulge this view.

My rule of thumb is that any method function that doesn't use its
_self_ argument to get at things gets made into a normal module level
function. This reduces clutter on the class (and in the source code)
and makes what's really going on clear to me and any later readers.

Of course there are some exceptions, such as:

* the function is only sometimes _self_-less; some implementations
  have to use _self_.
* the function varies on a per-class basis. This is a good case for
  StaticMethodUse.
* the function is a generic interface to multiple classes of objects,
  especially when they're spread out over multiple modules.

For me, most of the functions that wind up being de-method-ized this
way are internal helper functions. If a bunch of public interface
functions turn out to not use _self_, it's usually because my design
of the object's interface is bad.