A group of flags.
This corresponds only to the sequence of flags themselves, e.g.,
The span of this group of flags.
A sequence of flag items. Each item is either a flag or a negation operator.
Add the given item to this sequence of flags.
If the item was added successfully, then
None is returned. If the
given item is a duplicate, then
Some(i) is returned, where
items[i].kind == item.kind.
othervalues to be equal, and is used by