The last term in the equation ( ) is just a product of occupation number operators NA = a λ λ , NB = b µ µ of monomers A and B. In the remaining two terms partially self-contracted operators a λ 1 λ 2 κ 1 λ 1 and b µ 1 µ 2 ν 1 µ 1 appear. They can be rewritten as