def inject_simultaneous_muts(phi, dt, x, dx, theta):
    """
    simultaneous mutation model - see Hodgkinson and Eyre-Walker 2010, injected at (Delta,Delta)
    """
    phi[1, 1] += 1.0 / x[1] / x[1] / dx[1] / dx[1] * dt * theta
    return phi
