def set(config, key, value, sourceid="defaults"):
        """Sets a value in this config object *without* marking any source
        dirty, and with exact control of exactly where to set the
        value. This is mostly useful for low-level trickery with
        config objects.

        :param config: The configuration object to set values on
        :param key: The parameter name
        :param value: The new value
        :param sourceid: The identifier for the underlying source that the
                         value should be set on.
        """
        for source in config._sources:
            if source.identifier == sourceid:
                source.set(key, value)