Returns the next bookmark ID from a single counter shared by every bookmark.
Bookmark IDs must be unique within a document, but a bookmark is created
before it belongs to one, so all bookmarks draw from this one counter rather
than a generator per instance.
Returns
A number no earlier call has returned
Example
constfirst = bookmarkUniqueNumericId(); constsecond = bookmarkUniqueNumericId(); // first + 1
Returns the next bookmark ID from a single counter shared by every bookmark.
Bookmark IDs must be unique within a document, but a bookmark is created before it belongs to one, so all bookmarks draw from this one counter rather than a generator per instance.