api stability rules

[Johan Dahlin]

> What I propose is:
> 
>  * we try to give __names to make things genuinely private when they can
> be

That is discouraged by PEP-8:

"Python mangles these names with the class name: if class Foo has an
attribute named __a, it cannot be accessed by Foo.__a.  (An insistent
user could still gain access by calling Foo._Foo__a.)  Generally, double
leading underscores should be used only to avoid name conflicts with
attributes in classes designed to be subclassed."

Johan