Program construction by parts consists in tackling a complex specification one component at a time, developing a partially defined solution for each component, then combining the partial solutions into a global solution for the aggregate specification. This method is desirable whenever the specification at hand is too complex to be grasped in all its detail. It is feasible whenever the specification at hand is structured as an aggregate of clearly defined subspecifications ---where each subspecification represents a simple functional requirement. Our approach is based on relational specifications, whereby a specification is described by a binary relation. The set of relational specifications is naturally ordered by the refinement ordering, which provides a lattice-like structure. The join of two specifications S and S 0 is the specification that carries all the functional features of S and all the functional features of S 0 . Complex specifications are naturally structured as the j...

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