Extensions of the set of character scalars and of the domain of the INDEX function lead to use of lexical keys for nonpositional indexing of APL arrays. Using keys derived from the items of an array provides an approach to content addressing. When such keys are used to address rows of a matrix representing a relation in a relational data base, they serve as primary keys. To represent a secondary key, the matrix must be partitioned into a vector of appropriate submatrices.
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J. Philip Benkard (1982) studied this question.
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