unsets either the nth bookmark, the next (+1) or
previous (-1) set bookmarks, the automatic (-) bookmark, or
all (*) bookmarks, making it as if they had never been set; see
SetBookmark. If no option is specified, n is assumed to be
zero. While you can unset the automatic bookmark ‘-’, it will be
reset automatically to the current position whenever a
command is issued. Each document’s valid bookmark designations are 0 to 9,
and the ‘-’ automatic bookmark.