A categorical proof of the statement given by the title is provided, in generalization of a result for topological spaces proved recently by Clementino, Hofmann and Tholen.
Theory and Applications of Categories, Vol. 8, 2001, No. 16, pp 457-464 http://www.tac.mta.ca/tac/volumes/8/n16/n16.dvi http://www.tac.mta.ca/tac/volumes/8/n16/n16.ps http://www.tac.mta.ca/tac/volumes/8/n16/n16.pdf ftp://ftp.tac.mta.ca/pub/tac/html/volumes/8/n16/n16.dvi ftp://ftp.tac.mta.ca/pub/tac/html/volumes/8/n16/n16.psTAC Home