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.
Moves @count characters 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.