Invariance: Applications

Chapter 1 introduced the main method for proving that an assertion p is an invariant of program P. This method is essentially an induction on the positions in the computation. In this chapter we also identified the creative step of finding an inductive assertion that strengthens a given candidate invariant as the most difficult task in proofs of invariance. Some heuristics, such as supplementing assertions by preconditions, were proposed as techniques that may aid in the construction of inductive assertions.