A theory is complete if it does not contain a contradiction, while all of its proper extensions do. In this paper, first we introduce a relative notion of syntactic completeness; then we prove that adding exceptions to a programming language can be done in such a way that the completeness of the language is not made worse. These proofs are formalized in a logical system which is close to the usual syntax for exceptions, and they have been checked with the proof assistant Coq.
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