def get_class_module_root(cls):
    """
    Resolve the top namespace of a class associated module.

    >>> from omtk.core.classCtrl import BaseCtrl
    >>> BaseCtrl.__module__
    'omtk.core.classCtrl'
    >>> get_class_module_root(BaseCtrl)
    'omtk'

    :param cls: A class definition to inspect.
    :return: A str instance representing the root module.
    """
    return next(iter(cls.__module__.split('.')), None)