A new and an old preprint available. The new preprint is: "Envelopes - notion and definiteness", Aarhus Preprint Series No. 12 (2003), http://www.imf.au.dk/publications/matppt/2003/12.pdf Abstract: We examine critically some of the existing descriptions of the envelope of a 1-parameter family of surfaces in 3-space. Using the formalism of Synthetic Differential Geometry, the characteristic curves (which make up the envelope) can be described as intersections of 1st order neighbour surfaces. In the classical literature, such a description is presented as having "heuristic" value, only. The old preprint is: "Postulated colimits and left exactness of Kan-extensions", Aarhus Preprint Series 1989/90 No. 9. It has never been published, but due to some recent interest in the results, I have digitalized this preprint; it can be obtained from http://home.imf.au.dk/kock/postulated.pdf My Home Page has been improved, and contains now links AND short abstracts for other of my recent articles, including the 2002 version of "Algebra of Principal Fibre Bundles, and Connections" http://www.imf.au.dk/publications/matppt/2003/13.pdf The address of the home page is http://home.imf.au.dk/kock/ Anders Kock