def _add_default_fields(ds, fields):
    """
    Add some default fields to rays to assure they can be processed correctly.
    """
    if ('gas', 'temperature') in ds.derived_field_list:
        fields.append(('gas', 'temperature'))
    if ('gas', 'H_nuclei_density') in ds.derived_field_list:
        fields.append(('gas', 'H_nuclei_density'))
    return fields
