This paper is about transformations of knowledge bases with the aid of an imperative programming language which is non-standard in the sense that it features conditions (in loops and selection statements) that are description logic (DL) formulas, and a non-deterministic assignment statement (a choice operator given by a DL formula). We sketch an operational semantics of the proposed programming language and then develop a matching Hoare calculus whose pre- and post-conditions are again DL formulas. A major difficulty resides in showing that the formulas generated when calculating weakest preconditions remain within the chosen DL fragment. In particular, this concerns substitutions whose result is not directly representable. We therefore explicitly add substitution as a constructor of the logic and show how it can be eliminated by an interleaving with the rules of a traditional tableau calculu
