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An important consideration in the design of programming systems for the management of large files of data is the method of treating hierarchical data (that is, data among which logical relationships exist at more than two levels). Recent systems have accomplished this by simply duplicating some essential item of data at several levels. Such duplication makes the storage of even small data bases inefficient; for large masses of data, storage becomes economically unfeasible. Other systems have provided the means to specify and construct hierarchies, but none have provided language that affords control over retrieval and output levels, and control over the scope of output.
Robert E. Bleier (Sun,) studied this question.