This paper reports on the use of formal specifications in the development of a software maintenance tool for specializing imperative programs, which have become very complex due to extensive modifications. The tool is specified in terms of inference rules and operates by induction on the abstract syntax. The correctness of these rules is proved using rule induction. A Prolog prototype has been derived for Fortran programs, using the Centaur programming environment. Keywords: structured operational semantics, VDM, software maintenance, program specialization, proof of correctness, rule induction, Centaur.