  A combinatorial formula of G.-C. Rota and J.A. Stein is taken to perform Wick
re-ordering in quantum field theory. Wick's theorem becomes a Hopf algebraic
identity called Cliffordization. The combinatorial method relying on Hopf
algebras is highly efficient in computations and yields closed algebraic
expressions.
  AMS Subject Classifications 2000: 81R50; 16W30
