refined Graph implementation: more abstract/scalable Graph.Keys instead of plain lists -- order of adjacency is now standardized wrt. Key.ord;
@book{Birkhoff79,
author = {Garret Birkhoff},
title = {Lattice Theory},
publisher = {American Mathematical Society},
year=1979
}
@book{Nipkow-et-al:2002:tutorial,
author = {T. Nipkow and L. C. Paulson and M. Wenzel},
title = {{Isabelle/HOL} --- A Proof Assistant for Higher-Order Logic},
series = {LNCS},
volume = 2283,
year = 2002,
publisher = {Springer-Verlag}
}