If no record in T has key value x, return an error. Let's say that you're developing commercial software for CD cataloguing, and as part of the software you need a dictionary. You implement my dictionary with an array. We do not detail them here. New ADTs can be put in class libraries, and ADTs that others have created can be found in class libraries.
Class libraries make programming easier through code reuse.
Skip Sections 7.10 and 7.11.