No, I would consider this an implementation detail.  "Users" aren't really supposed to use it at all; only people implementing new fitters. Of course users might sometimes have need to implement new fitters so this is one of those areas where the line between users and developers is blurred.  Nevertheless I would still consider this an implementation detail that is subject to change.  Going forward we need to think about an easier interface for creating "user-defined" fitters a la `custom_model` (though there are some aspects of this in the case of fitters that is inherently more complicated :(