more on locally closed sublocales and subtoposes

This is to announce the preprint A. Kock and T. Plewe: Locally closed subtoposes, and prolongation by zero. It is available by anonymous ftp from theory.doc.ic.ac.uk/papers/Kock where it appears as the file Kock-Plewe.ps or Kock-Plewe.dvi . It subsumes the paper by Kock, announced here on Feb. 15 1996, and this paper has therefore now been removed from the above ftp-site. Abstract: We prove the equivalence of some conditions on a complemented subtopos of a topos, one of which is that the subtopos is locally closed, and another is that abelian group objects in the smaller topos admit prolongation by zero; the prolongation functor then has a right adjoint.