Moves count characters 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 new position of this is different from its original position,
and dereferenceable (the last iterator in the buffer is not dereferenceable). If count is 0, the function does nothing and
returns false.
| this |
an iterator |
| count |
number of characters to move, may be negative |
|
whether this moved and is dereferenceable |