count visible lines backward, if possible.
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
count is negative, moves forward by 0 -
number of lines to move backward
whether this moved and is dereferenceable