Software contracts take the form of routine preconditions, postconditions, and class invariants written into the program itself. The design by contract methodology uses such contracts for building each software element, an approach that is particularly appropriate for developing safety-critical software and reusable libraries. This methodology is a key design element of some existing libraries, especially the Eiffel software development environment, which incorporates contract mechanisms in the programming language itself. Because the authors see the contract metaphor as inherent to quality software development, they undertook the work reported here as a sanity check to determine whether they see contracts everywhere simply because their development environment makes using them natural or whether contracts are intrinsically present, even when other designers don't express or even perceive them.
