where the operator ϕ ϕ i ( x 1 ) ϕ ϕ i ( x 2 ) ¯ ¯ are the amount that do not annihilate the vacuum state | 0 ⟩ ⟩ . Which means that A B ¯ ¯ = T A B − − : T A B : . This means that A B ¯ ¯ is a contraction over T A B . Note that the contraction of a time-ordered string of two field operators is a C-number.