Part of an HL7 message: either a segment or group. There are certain methods (e.g. Group.get())
that will always return either a segment or a group. This interface allows methods like this
to declare a return type of Structure instead of Object.
Returns the Message object to which this structure belongs. This should normally be set at
construction time. A Structure can only belong to a single Message. This is primarily
to avoid a situation where intended changes in one message cause unintended changes
in another that shares one of the same Structure objects.