Equation Icon  ({ i [ ] ′ -- ˆA(0)(t), ˆx(0)(t′) , t ≥ t′ χAx(t,t) ≡ ( ℏ ′ 0, t < t ({ i [ ] ′ -- ˆB (0)(t),Fˆ(0)(t′) , t ≥ t′ χBF (t,t) ≡ ( ℏ ′ . (127 ) 0, t < t