  We introduce a novel characterization of all Walrasian price vectors in terms
of forbidden over- and under demanded sets for monotone gross substitute
combinatorial auctions.
  For ascending and descending auctions we suggest a universal framework for
finding the minimum or maximum Walrasian price vectors for monotone gross
substitute combinatorial auctions. An ascending (descending) auction is
guaranteed to find the minimum (maximum) Walrasian if and only if it follows
the suggested framework.
