Stepwise Refinement

Stepwise refinement is the operational procedure behind the principle that elegance and correctness should be built into a program from the first line. The programmer does not write a complex system and then try to verify it. She starts with an abstract specification, derives a first version that is as simple as the specification permits and provably correct at that level of abstraction, and then refines the version step by step — each step small enough to be verified, each step preserving the properties the previous steps established. The finished program is complex only to the extent the problem required complexity; every piece of the complexity was introduced deliberately, and every introduction preserved the correctness of the whole.