As is_subfeature_of, for subproperties.

def __is_subproperty_of (parent_property, p):
    """ As is_subfeature_of, for subproperties.
    """
    if __debug__:
        from .property import Property
        assert isinstance(parent_property, Property)
        assert isinstance(p, Property)
    return is_subfeature_of (parent_property, p.feature)