def fiddle(schema):
    """Fiddle the schema.

        This is a copy of the class' schema, with any ISchemaExtender-provided
        fields added. The schema may be modified in-place: there is no
        need to return a value.

        In general, it will be a bad idea to delete or materially change
        fields, since other components may depend on these ones.

        If you change any fields, then you are responsible for making a copy of
        them first and place the copy in the schema.
        """
