Invariance of Gromov-Witten theory under a simple flop

Let X be a smooth complex projective manifold and psi : X → X a flopping contraction in the sense of minimal model theory, with psi : Z ≅ Pr → pt the restriction map to the extremal contraction. Assume that NZ/X ≅ OPr (-1)⊕(r +1). Then a simple Pr flop f : X → X' exists. We show that the generating functions of Gromov-Witten invariants with ancestors are invariant under a simple flop, for all genera.