def describe_diffs_in_values(df_obs, df_exp, mask, name='', n_show=5):
    """ Show first few rows that differ between two dataframes, according to boolean mask."""
    df_prev_obs = df_obs.loc[mask].head(n_show)
    df_prev_exp = df_exp.loc[mask].head(n_show)
    return '\n'.join([f'{name} are different: first few rows of diffs:',
                      'observed', repr(df_prev_obs), 'expected', repr(df_prev_exp)])