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Sun's Link Service, a product shipped with Sun's programming in the large software development environment, the Network Software Environment, allows users to make and maintain explicit and persistent bidirectional relationships between autonomous front end applications. The Link Service defines a protocol for an extensible and loosely coupled, or open, hypertext system. An interesting instance of this is the ability to link to objects in closed hypertext systems if they integrate with the Link Service. The Link Service addresses link maintenance and automated versioning. Link endpoints, or nodes, are defined by the integrating applications, and are not restricted to points, whole documents, or cards.
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Arthur Pearl (1989) studied this question.