Generate FORCE_SETS string

    See the format of dataset in the docstring of
    Phonopy.set_displacement_dataset. Optionally for the type-1 (traditional)
    format, forces can be given. In this case, sets of forces are
    unnecessary to be stored in the dataset.

def get_FORCE_SETS_lines(dataset, forces=None):
    """Generate FORCE_SETS string

    See the format of dataset in the docstring of
    Phonopy.set_displacement_dataset. Optionally for the type-1 (traditional)
    format, forces can be given. In this case, sets of forces are
    unnecessary to be stored in the dataset.

    """

    if 'first_atoms' in dataset:
        return _get_FORCE_SETS_lines_type1(dataset, forces=forces)
    elif 'forces' in dataset:
        return _get_FORCE_SETS_lines_type2(dataset)