The preprint Algebra of Principal Fibre Bundles, and Connections is available at ftp://ftp.imf.au.dk/pub/kock/princ4.ps (102 kb). The classical relationship between the curvature of a connection, and the coboundary of its connection form, here comes about from a pure groupoid calculation. The preprint updates and expands my 1983/1986 paper, "Combinatorial notions relating to principal fibre bundles". Anders Kock http://www.imf.au.dk/~kock/