You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: to_lowercase.rs
    path of file: ./repos/nushell/crates/nu-cmd-dataframe/src/dataframe/series/string
    
    the code of the file until where you have to start completion: use super::super::super::values::{Column, NuDataFrame};

use nu_protocol::{
    ast::Call,
    engine::{Command, EngineState, Stack},
    Category, Example, PipelineData, ShellError, Signature, Span, Type, Value,
};
use polars::prelude::{IntoSeries, StringNameSpaceImpl};

#[derive(Clone)]
pub struct ToLowerCase;

impl Command for ToLowerCas
    
    fn examples(&self) -> Vec<Example> {
        vec![Example {
            description: "Modifies strings to lowercase",
            example: "[Abc aBc abC] | dfr into-df | dfr lowercase",
            result: Some(
                NuDataFrame::try_from_columns(
                    vec![Column::new(
                        "0".to_string(),
                        vec![
                            Value::test_string("abc"),
                            Value::test_string("abc"),
                            Value::test_string("abc"),
                        ],
                    )],
                    None,
                )
                .expect("simple df for test should not fail")
                .into_value(Span::test_data()),
            ),
        }]
    }