Moves @count visible lines backward, if possible (if @count would move
past the start or end of the buffer, moves to the start or end of
the buffer). The return value indicates whether the iterator moved
onto a dereferenceable position; if the iterator didn’t move, or
moved onto the end iterator, then %FALSE is returned. If @count is 0,
the function does nothing and returns %FALSE. If @count is negative,
moves forward by 0 - @count lines.
Moves @count visible lines backward, if possible (if @count would move past the start or end of the buffer, moves to the start or end of the buffer). The return value indicates whether the iterator moved onto a dereferenceable position; if the iterator didn’t move, or moved onto the end iterator, then %FALSE is returned. If @count is 0, the function does nothing and returns %FALSE. If @count is negative, moves forward by 0 - @count lines.