code: def set(config, key, value, sourceid="defaults"):
          
          for source in config._sources:
              if source.identifier == sourceid:
                  source.set(key, value)
docs: 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.