def _inject_mutations_2D(phi, dt, xx, yy, theta0, frozen1, frozen2, nomut1,
    nomut2):
    """
    Inject novel mutations for a timestep.
    """
    if not frozen1 and not nomut1:
        phi[1, 0] += dt / xx[1] * theta0 / 2 * 4 / ((xx[2] - xx[0]) * yy[1])
    if not frozen2 and not nomut2:
        phi[0, 1] += dt / yy[1] * theta0 / 2 * 4 / ((yy[2] - yy[0]) * xx[1])
    return phi
