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: flatten.rs
    path of file: ./repos/nushell/crates/nu-cmd-dataframe/src/dataframe/lazy
    
    the code of the file until where you have to start completion: use nu_protocol::{
    ast::Call,
    engine::{Command, EngineState, Stack},
    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
};

use crate::dataframe::values::{Column, NuDataFrame};

use super::explode::explode;

#[derive(Clone)]
pub struct LazyFlatten;

impl Command for LazyFlatten {
    fn name(&self) -> &str {
        "dfr flatten"
    }

    fn usage(&self) -> &str {
        "An alias for dfr explode."
    }

    fn signature(&self) -> Signature {
        Signature::build(self.name())
            .rest(
                "columns",
                SyntaxShape::String,
                "columns to flatten, only applicable for dataframes",
            )
            .input_output_types(vec![
                (
                    Type::Custom("expression".into()),
                    Type::Custom("expression".into()),
                ),
                (
                    Type::Custom("dataframe".into()),
                    Type::Custom("dataframe".into()),
                ),
            ])
            .category(Category::Custom("lazyframe".into()))
    }

    fn examples(&self) -> Vec<Example> {
        vec![
Example {
                description: "Flatten the specified dataframe",
                example: "[[id name hobbies]; [1 Mercy [Cycling Knitting]] [2 Bob [Skiing Football]]] | dfr into-df | dfr flatten hobbies | dfr collect",
                result: Some(
                   NuDataFrame::try_from_columns(vec![
                    Column::new(
                        "id".to_string(), 
                        vec![
                            Value::test_int(1),
                            Value::test_int(1),
                            Value::test_int(2),
                            Value::test_int(2),
                        ]),
                    Column::new(
                        "name".to_string(), 
                        vec![
                            Value::test_string("Mercy"),
                            Value
    
    fn examples(&self) -> Vec<Example> {
        vec![
Example {
                description: "Flatten the specified dataframe",
                example: "[[id name hobbies]; [1 Mercy [Cycling Knitting]] [2 Bob [Skiing Football]]] | dfr into-df | dfr flatten hobbies | dfr collect",
                result: Some(
                   NuDataFrame::try_from_columns(vec![
                    Column::new(
                        "id".to_string(), 
                        vec![
                            Value::test_int(1),
                            Value::test_int(1),
                            Value::test_int(2),
                            Value::test_int(2),
                        ]),
                    Column::new(
                        "name".to_string(), 
                        vec![
                            Value::test_string("Mercy"),
                            Value::test_string("Mercy"),
                            Value::test_string("Bob"),
                            Value::test_string("Bob"),
                        ]),
                    Column::new(
                        "hobbies".to_string(), 
                        vec![
                            Value::test_string("Cycling"),
                            Value::test_string("Knitting"),
                            Value::test_string("Skiing"),
                            Value::test_string("Football"),
                        ]),
                   ], None).expect("simple df for test should not fail")
                   .into_value(Span::test_data()),
                    )
            },
        Example {
                description: "Select a column and flatten the values",
                example: "[[id name hobbies]; [1 Mercy [Cycling Knitting]] [2 Bob [Skiing Football]]] | dfr into-df | dfr select (dfr col hobbies | dfr flatten)",
                result: Some(
                   NuDataFrame::try_from_columns(vec![
                    Column::new(
                        "hobbies".to_string(), 
                        vec![
                            Value::test_string("Cycling"),
                            Value::test_string("Knitting"),
                            Value::test_string("Skiing"),
                            Value::test_string("Football"),
                        ]),
                   ], None).expect("simple df for test should not fail")
                   .into_value(Span::test_data()),
                    ),
            },
        ]
    }