  Since quite a time there were available only two rather difficult and
involved proofs, the original one by Arveson and a more recent one by
Liebscher, of the fact that for every Arveson system there exists an
E_0-semigroup. We put together two recent short proofs, one by Skeide and one
by Arveson, to obtain a still simpler one, which unfies the advantages of each
proof and discards with their disadvantages.
