@article(
MitchExp,author="Mitchell, J.C", 
        Title="On abstraction and the expressive power of 
programming languages",
        journal="Science of Computer Programming",
        Volume="212",Year="1993", pages="141--163",
	Note="Preliminary version appeared {\it Proc. Theor. Aspects of Computer Software,}
        Springer LNCS 526, September 1991, pages 750--770.")