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: snake_case.rs
    path of file: ./repos/nushell/crates/nu-cmd-extra/src/extra/strings/str_/case
    
    the code of the file until where you have to start completion: use heck::ToSnakeCase;
use nu_protocol::ast::Call;
use nu_protocol::engine::{Command, EngineState, Stack};
use nu_protocol::{
    record, Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
};

use super::operate;
#[derive(Clone)]
pub struct SubCommand;

impl Command for SubCommand {
    fn name(&self) -> &str {
        "str snake-case"
    }

    fn signature(&self) -> Signature {
        Signature::build("str snake-case")
            .input_output_types(vec![
                (Type::String, T
    
    fn examples(&self) -> Vec<Example> {
        vec![
            Example {
                description: "convert a string to snake_case",
                example: r#" "NuShell" | str snake-case"#,
                result: Some(Value::test_string("nu_shell")),
            },
            Example {
                description: "convert a string to snake_case",
                example: r#" "this_is_the_second_case" | str snake-case"#,
                result: Some(Value::test_string("this_is_the_second_case")),
            },
            Example {
                description: "convert a string to snake_case",
                example: r#""this-is-the-first-case" | str snake-case"#,
                result: Some(Value::test_string("this_is_the_first_case")),
            },
            Example {
                description: "convert a column from a table to snake_case",
                example: r#"[[lang, gems]; [nuTest, 100]] | str snake-case lang"#,
                result: Some(Value::test_list(vec![Value::test_record(record! {
                    "lang" => Value::test_string("nu_test"),
                    "gems" => Value::test_int(100),
                })])),
            },
        ]
    }