Prolongation by zero (preprint)

The preprint "Locally closed sublocales and prolongation by zero" is available by anonymous ftp from theory.doc.ic.ac.uk/papers/Kock where it appears as the file: loc-clo.ps (it is about 150 kb). Abstract: We prove that abelian group valued sheaves over a locally closed sublocale admit prolongation by zero, and that the prolongation functor has a right adjoint. The method is by left exact comonads; specifically, it uses a version of Artin-Wraith glueing, where one has to put glue on _both_ the items to be glued.