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\section*{Acknowledgements} 
First of all we would like to thank J.H.~K\"uhn for his kind advice and
numerous valuable suggestions and discussions.  We are grateful to the
following colleagues for carefully reading the manuscript (or parts of
it) and their expert advice on various subjects: P.~Baikov,
K.G.~Chetyrkin, P.~Gambino, T.~Hahn, W.~Kilian, B.A.~Kniehl,
K.~Melnikov, A.~Pukhov, A.~R\'etey, T.~van~Ritbergen, T.~Seidensticker,
V.A.~Smirnov, R.G.~Stuart, M.~Stuber, O.V.~Tarasov, and G.~Weiglein.
This work was supported in part by {\it Graduiertenkolleg
  ``Elementarteilchenphysik an Beschleunigern''} and {\it
  DFG-Forschergruppe ``Quantenfeldtheorie, Computeralgebra und
  Monte-Carlo-Simulationen''} at the University of Karlsruhe, and the
{\it Schweizer Nationalfond}.

