def laplacian(field, m, s, kernel):
    """
    Spacial discretization for the isotropic acoustic wave equation. For a 4th
    order in time formulation, the 4th order time derivative is replaced by a
    double laplacian:
    H = (laplacian + s**2/12 laplacian(1/m*laplacian))
    :param field: Symbolic TimeFunction object, solution to be computed
    :param time_order: time order
    :param m: square slowness
    :param s: symbol for the time-step
    :return: H
    """
    biharmonic = field.laplace2(1/m) if kernel == 'OT4' else 0
    return field.laplace + s**2/12 * biharmonic