def get_overridden_methods(cls):
    """
    Returns a set of all overridden methods of the class ``cls``.
    """
    # collect all attributes inherited from parent classes
    parent_attrs = set()
    for base in cls.__bases__:
        parent_attrs.update(dir(base))
    
    # find all methods implemented in the class itself
    methods = {name for name, thing in vars(cls).items() if callable(thing)}
    
    # return the intersection of both
    return parent_attrs.intersection(methods)