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As a counterpart of the well-known generalized Wick theorem by Bais et. al.in 1988 for interacting fields in two dimensional conformal field theory, wepresent a new contour integral formula for the operator product expansion of anormally ordered operator and a single operator on its right hand. Quitesimilar to the original Wick theorem for the opposite order operator product,it expresses the contraction i.e.the singular part of the operator productexpansion as a contour integral of only two terms, each of which is a productof a contraction and a single operator. We discuss the relation between theseformulas and the Borcherds identity satisfied by the quantum fields associatedwith the theory of vertex algebras.

Generalized Wick theorems in conformal field theory and the Borcherds identity