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: expression.rs
    path of file: ./repos/toydb/src/sql/types
    
    the code of the file until where you have to start completion: use super::{Row, Value};
use crate::error::{Error, Result};

use regex::Regex;
use serde_derive::{Deserialize, Serialize};
use std::fmt::{self, Display};
use std::mem::replace;

/// An expression, made up of constants and operations
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub enum Expression {
    // Values
    Constant(Value),
    Field(usize, Option<(Option<String>, String)>),

    // Logical operations
    And(Box<Expression>, Box<Expression>),
    Not(Box<Expression>),
    Or(Box<Expression>, Box<Expression>),

    // Comparisons operations (GTE, LTE, and NEQ are composite 
    
    pub fn into_dnf(self) -> Self {
        use Expression::*;
        self.into_nnf()
            .transform(
                &|e| match e {
                    And(lhs, rhs) => match (*lhs, *rhs) {
                        (Or(ll, lr), r) => {
                            Ok(Or(And(ll, r.clone().into()).into(), And(lr, r.into()).into()))
                        }
                        (l, Or(rl, rr)) => {
                            Ok(Or(And(l.clone().into(), rl).into(), And(l.into(), rr).into()))
                        }
                        (lhs, rhs) => Ok(And(lhs.into(), rhs.into())),
                    },
                    e => Ok(e),
                },
                &Ok,
            )
            .unwrap()
    }