The stimulating discussions with Mike Codish;
Patrick Cousot;
Saumya Debray and Giorgio Levi are gratefully acknowledged. This work has been done while the author was visiting the LJX;
Laborutoire d'Informatique;
£cole Polytechnique;
Paris. He thanks LJX;
and in particular Radhia Cousot;
for the kind hospitality. This work has been partly supported by the EEC Human Capital and Mobility individual grant: 'Semantic Definitions;
Abstract Interpretation and Constraint Reasoning';
N;
ERBCHBICT930822;