def subscribe(observer):
    """Register a callable that will be invoked (with a single
        IBuildControl object) for each Build that is created to satisfy this
        request. There may be multiple Builds created in an attempt to handle
        the request: they may be interrupted by the user or abandoned due to
        a lost worker. The last Build (the one which actually gets to run to
        completion) is said to 'satisfy' the BuildRequest. The observer will
        be called once for each of these Builds, both old and new."""
