'Add an annotation (:class:`BaseSchemeAnnotation` subclass) instance
to the scheme.'

def add_annotation(self, annotation):

    check_arg((annotation not in self.__annotations), 'Cannot    add    the    same    annotation    multiple    times.')
    check_type(annotation, BaseSchemeAnnotation)
    self.__annotations.append(annotation)
    self.annotation_added.emit(annotation)
