diff --git a/src/expr.rs b/src/expr.rs
index fd05385..0c65af8 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -3,11 +3,11 @@ use miette::{miette, Context, Result};
 use std::collections::HashMap;
 
 #[derive(Debug, Clone)]
-pub enum Expr {
-    Literal(String),
-    Not(Box<Expr>),
-    And(Vec<Expr>),
-    Or(Vec<Expr>),
+pub enum Expr<'s> {
+    Literal(&'s str),
+    Not(Box<Expr<'s>>),
+    And(Vec<Expr<'s>>),
+    Or(Vec<Expr<'s>>),
 }
 
 #[derive(Debug)]
@@ -88,11 +88,11 @@ impl<'s> Iterator for Tokanizer<'s> {
     }
 }
 
-impl Expr {
-    fn parse_expr(tokanizer: &mut Tokanizer) -> Result<Expr> {
+impl<'s> Expr<'s> {
+    fn parse_expr(tokanizer: &mut Tokanizer<'s>) -> Result<Expr<'s>> {
         if let Some(token) = tokanizer.next().transpose()? {
             match token {
-                Token::Literal(l) => Ok(Expr::Literal(l.to_string())),
+                Token::Literal(l) => Ok(Expr::Literal(l)),
                 Token::Not => Ok(Expr::Not(Box::new(Self::parse_expr(tokanizer)?))),
                 Token::ParenOpen => {
                     let first_expr = Self::parse_expr(tokanizer)?;
@@ -157,7 +157,7 @@ impl Expr {
         }
     }
 
-    pub fn parse(input: &str) -> Result<Self> {
+    pub fn parse(input: &'s str) -> Result<Self> {
         let mut v = Vec::new();
         for (i, line) in input.lines().enumerate() {
             let mut tokanizer = Tokanizer::new(line);
@@ -275,7 +275,7 @@ impl Expr {
         }
     }
 
-    pub fn to_dpll_cnf(self) -> (Vec<Vec<Index>>, HashMap<String, Index>) {
+    pub fn to_dpll_cnf(self) -> (Vec<Vec<Index>>, HashMap<&'s str, Index>) {
         let mut map = HashMap::new();
 
         let mut cnf = Vec::new();