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: functions.rs
    path of file: ./repos/polars/crates/polars-lazy/src/dsl
    
    the code of the file until where you have to start completion: //! # Functions
//!
//! Function on multiple expressions.
//!
use polars_core::prelude::*;
pub use polars_plan::dsl::functions::*;
use rayon::prelude::*;

use crate::prelude::*;

pub(crate) fn concat_impl<L: AsRef<[LazyFrame]>>(
    inputs: L,
    rechunk: bool,
    parallel: bool,
    from_partitioned_ds: bool,
    convert_supertypes: bool,
) -> PolarsResult<LazyFrame> {
    let mut inputs = inputs.as_ref().to_vec();

    let mut lf = std::mem::take(
        inputs
            .get_mut(0)
            .ok_or_else(|| polars_err!(NoData: "empty container given"))?,
 
    
    fn test_diag_concat_lf() -> PolarsResult<()> {
        let a = df![
            "a" => [1, 2],
            "b" => ["a", "b"]
        ]?;

        let b = df![
            "b" => ["a", "b"],
            "c" => [1, 2]
        ]?;

        let c = df![
            "a" => [5, 7],
            "c" => [1, 2],
            "d" => [1, 2]
        ]?;

        let out = concat_lf_diagonal(
            &[a.lazy(), b.lazy(), c.lazy()],
            UnionArgs {
                rechunk: false,
                parallel: false,
                ..Default::default()
            },
        )?
        .collect()?;

        let expected = df![
            "a" => [Some(1), Some(2), None, None, Some(5), Some(7)],
            "b" => [Some("a"), Some("b"), Some("a"), Some("b"), None, None],
            "c" => [None, None, Some(1), Some(2), Some(1), Some(2)],
            "d" => [None, None, None, None, Some(1), Some(2)]
        ]?;

        assert!(out.equals_missing(&expected));

        Ok(())
    }