It is profitable to view the process of writing programs as an engineering activity. A program is a deliberately contrived mechanism constructed from parts whose behaviors are combined to produce the behavior of the whole. We propose to develop a notion of understanding a program which is analogous to similar notions in other engineering subjects. Understanding is a rich notion in engineering domains. It includes the ability to identify the parts of a mechanism and assign a purpose to each part. Understanding also entails being able to explain to someone how a mechanism works and rationalize its behavior under unusual circumstances.
