[1] Department of Computer Science,
[2] Chalmers University of Technology and Gothenburg University,undefined
[3] SE-412 96 Göteborg,undefined
[4] Sweden. e-mail: coquand@cs.chalmers.se,undefined
[5] Department of Mathematics,undefined
[6] Uppsala University,undefined
[7] P.O. Box 480,undefined
[8] SE-751 06 Uppsala,undefined
[9] Sweden. e-mail: palmgren@math.uu.se,undefined