ing each object, the marker is invoked to mark all objects
reachable from it.  The object itself is not explicitly marked.
This assures that objects on which a finalizer depends are neither
collected nor finalized.
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If in the process of marking from an object the
object itself becomes marked, we have uncovered
a cycle involving the object.  This usually results in a warning from the
collector.  Such objects are not finalized, since it may be
unsafe to do so.  See the more detailed
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