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Reliability of the Solution

• Why should I believe that your program is correct?
  • Formal model of the specification.
  • Designed model system such that some invariants are maintained.
  • Can reason that invariants imply specification.
  • Code is implementation of the model system.
• But you have not verified your allegations in detail.
  • With more efforts (costs), reasoning can be refined.
  • Show that model system indeed maintains invariants.
  • Refine invariants such that they indeed imply specification.
  • Show that implementation preserves properties of model.

Design process increases confidence and supports further analysis.