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: mod.rs
    path of file: ./repos/nushell/crates/nu-cmd-dataframe/src/dataframe/values/nu_when
    
    the code of the file until where you have to start completion: mod custom_value;

use core::fmt;
use nu_protocol::{ShellError, Span, Value};
use polars::prelude::{col, when, ChainedThen, Then};
use serde::{Deserialize, Deserializer, Serialize, Serializer};

#[derive(Clone)]
pub enum NuWhen {
    Then(Box<Then>),
    ChainedThen(ChainedThen),
}

// Mocked serialization of the LazyFrame object
impl Serialize for NuWhen {
    fn seria
    
    pub fn try_from_value(value: Value) -> Result<Self, ShellError> {
        let span = value.span();
        match value {
            Value::CustomValue { val, .. } => match val.as_any().downcast_ref::<Self>() {
                Some(expr) => Ok(expr.clone()),
                None => Err(ShellError::CantConvert {
                    to_type: "when expression".into(),
                    from_type: "non when expression".into(),
                    span,
                    help: None,
                }),
            },
            x => Err(ShellError::CantConvert {
                to_type: "when expression".into(),
                from_type: x.get_type().to_string(),
                span: x.span(),
                help: None,
            }),
        }
    }