Two Views of Data Abstraction CACM June, 1977 Ledgard, H. F. Taylor, R. W. CA770603 JB December 29, 1977 1:28 AM 2960 5 2960 2960 5 2960 2960 5 2960 3049 5 2960 2956 6 2960 2958 6 2960 2957 6 2960 2960 6 2960