  For a compact set, we characterize the existence of a linear extension
operator E for the space of Whitney jets without loss of derivatives, that is,
E satisfies the best possible continuity estimates: The supremum of all partial
derivatives up to order n of E(f) is less or equal than a constant times the
n-th Whitney norm of f. The characterization is a surprisingly simple purely
geometric condition telling in a way that at all its points, the set is big
enough in all directions.
