  Matrix code allows one to discover algorithms and to render them in code that
is both compilable and is correct by construction. In this way the difficulty
of verifying existing code is avoided. The method is especially important for
logically dense code and when precision programming is called for. The paper
explains both these concepts. Logically dense code is explained by means of the
partition stage of the Quicksort algorithm. Precision programming is explained
by means of fast exponentiation.
