Generalization of Wick's Theorem

Question

Wick's theorem allow us to write a time-ordering of creation and annihilation operators as a normal-ordering of contractions of these operators. I am studying a system that consists of two kinds of fermions (electrons and positrons) and I have creation and annihilation operators for both. In general, Wick's theorem is applied to systems with one kind of a particle. I would like to know if there is some generalization of the theorem to my case. My difficult is to determine what would be a contraction of a positron creation or annihilation operator with an electron operator, and if the definition of the normal-ordering doesn't change.