The header in "bookmark.vim" was pretty weak. It provided no description
of the class it contains and no direction for the reader. In particular
it did not note the dual purpose of the "Bookmark" class.
The fact that the "Bookmark" class serves two purposes must be noted
because many readers will expect class definitions to obey the single
responsibility principle!
If there is a chance for a major refactor of this class in the future, a
priority would be splitting the class in two so that a "BookmarkList"
class can assume the responsibility for providing a container for all
"Bookmark" objects.
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