diff --git a/src/builder.rs b/src/builder.rs
index 47aca04..85bea25 100644
--- a/src/builder.rs
+++ b/src/builder.rs
@@ -73,88 +73,200 @@ impl CpModelBuilder {
             vars: vars.into_iter().map(|v| v.0).collect(),
         }))
     }
+
+    /// Add a linear constraint
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::CpModelBuilder;
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(0, 100)]);
+    /// let y = model.new_int_var([(0, 100)]);
+    /// model.add_linear_constraint([(1, x), (3, y)], [(301, i64::MAX)]);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) + 3 * y.solution_value(&response) >= 301);
+    /// ```
     pub fn add_linear_constraint(
         &mut self,
-        expr: &LinearExpr,
-        (begin, end): (i64, i64),
+        expr: impl Into<LinearExpr>,
+        domain: impl IntoIterator<Item = (i64, i64)>,
     ) -> Constraint {
+        let expr = expr.into();
+        let constant = expr.constant;
         self.add_cst(CstEnum::Linear(proto::LinearConstraintProto {
-            vars: expr.vars.clone(),
-            coeffs: expr.coeffs.clone(),
-            domain: vec![begin - expr.constant, end - expr.constant],
+            vars: expr.vars,
+            coeffs: expr.coeffs,
+            domain: domain
+                .into_iter()
+                .flat_map(|(begin, end)| {
+                    [
+                        if begin == i64::MIN {
+                            i64::MIN
+                        } else {
+                            begin.saturating_sub(constant)
+                        },
+                        if end == i64::MAX {
+                            i64::MAX
+                        } else {
+                            end.saturating_sub(constant)
+                        },
+                    ]
+                })
+                .collect(),
         }))
     }
+
+    /// Add an equality constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(0, 50)]);
+    /// let y = model.new_int_var([(53, 100)]);
+    /// model.add_eq(y, LinearExpr::from(x) + 3);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert_eq!(y.solution_value(&response), x.solution_value(&response) + 3);
+    /// assert_eq!(50, x.solution_value(&response));
+    /// assert_eq!(53, y.solution_value(&response));
+    /// ```
     pub fn add_eq<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_eq_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_eq_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([val, val]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(0, 0)])
     }
+
+    /// Add a greater or equal constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(0, 50)]);
+    /// let y = model.new_int_var([(50, 100)]);
+    /// model.add_ge(x, y);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) >= y.solution_value(&response));
+    /// assert_eq!(50, x.solution_value(&response));
+    /// assert_eq!(50, y.solution_value(&response));
+    /// ```
     pub fn add_ge<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_ge_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_ge_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([val, i64::MAX]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(0, i64::MAX)])
     }
+
+    /// Add a lesser or equal constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(50, 100)]);
+    /// let y = model.new_int_var([(0, 50)]);
+    /// model.add_le(x, y);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) <= y.solution_value(&response));
+    /// assert_eq!(50, x.solution_value(&response));
+    /// assert_eq!(50, y.solution_value(&response));
+    /// ```
     pub fn add_le<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_le_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_le_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([i64::MIN, val]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(i64::MIN, 0)])
     }
+
+    /// Add a stricly greater constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(0, 51)]);
+    /// let y = model.new_int_var([(50, 100)]);
+    /// model.add_gt(x, y);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) > y.solution_value(&response));
+    /// assert_eq!(51, x.solution_value(&response));
+    /// assert_eq!(50, y.solution_value(&response));
+    /// ```
     pub fn add_gt<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_gt_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_gt_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([val + 1, i64::MAX]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(1, i64::MAX)])
     }
+
+    /// Add a strictly lesser constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(50, 100)]);
+    /// let y = model.new_int_var([(0, 51)]);
+    /// model.add_lt(x, y);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) < y.solution_value(&response));
+    /// assert_eq!(50, x.solution_value(&response));
+    /// assert_eq!(51, y.solution_value(&response));
+    /// ```
     pub fn add_lt<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_lt_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_lt_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([i64::MIN, val - 1]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(i64::MIN, -1)])
     }
+
+    /// Add a not equal constraint between 2 linear expressions.
+    ///
+    /// # Exemple
+    ///
+    /// ```
+    /// # use cp_sat::builder::{CpModelBuilder, LinearExpr};
+    /// # use cp_sat::proto::CpSolverStatus;
+    /// let mut model = CpModelBuilder::default();
+    /// let x = model.new_int_var([(50, 100)]);
+    /// let y = model.new_int_var([(0, 51)]);
+    /// model.add_lt(x, y);
+    /// let response = model.solve();
+    /// assert_eq!(response.status(), CpSolverStatus::Optimal);
+    /// assert!(x.solution_value(&response) < y.solution_value(&response));
+    /// assert_eq!(50, x.solution_value(&response));
+    /// assert_eq!(51, y.solution_value(&response));
+    /// ```
     pub fn add_ne<T: Into<LinearExpr>, U: Into<LinearExpr>>(
         &mut self,
         lhs: T,
         rhs: U,
     ) -> Constraint {
-        self.add_ne_by_ref(&lhs.into(), &rhs.into())
-    }
-    pub fn add_ne_by_ref(&mut self, lhs: &LinearExpr, rhs: &LinearExpr) -> Constraint {
-        let (mut cst, val) = create_linear_cst_proto(lhs, rhs);
-        cst.domain.extend([i64::MIN, val - 1, val + 1, i64::MAX]);
-        self.add_cst(CstEnum::Linear(cst))
+        self.add_linear_constraint(lhs.into() - rhs.into(), [(i64::MIN, -1), (1, i64::MAX)])
     }
     pub fn add_min_eq(
         &mut self,
@@ -192,7 +304,6 @@ impl CpModelBuilder {
     /// ```
     /// # use cp_sat::builder::CpModelBuilder;
     /// # use cp_sat::proto::{CpSolverStatus, SatParameters};
-    /// # use cp_sat::proto::sat_parameters::SearchBranching;
     /// let mut model = CpModelBuilder::default();
     /// let x = model.new_int_var([(0, 100)]);
     /// let y = model.new_bool_var();
@@ -201,7 +312,6 @@ impl CpModelBuilder {
     /// model.add_ge([(1, x), (3, y.into())], 3);
     /// model.maximize(y);
     /// let response = model.solve();
-    /// println!("{:#?}", response);
     /// assert_eq!(response.status(), CpSolverStatus::Optimal);
     /// ```
     pub fn add_hint(&mut self, var: impl Into<IntVar>, value: i64) {
@@ -311,6 +421,31 @@ pub struct LinearExpr {
     coeffs: Vec<i64>,
     constant: i64,
 }
+impl std::ops::AddAssign for LinearExpr {
+    fn add_assign(&mut self, mut rhs: Self) {
+        if self.vars.len() < rhs.vars.len() {
+            std::mem::swap(self, &mut rhs);
+        }
+        self.vars.extend_from_slice(&rhs.vars);
+        self.coeffs.extend_from_slice(&rhs.coeffs);
+        self.constant += rhs.constant;
+    }
+}
+impl std::ops::Neg for LinearExpr {
+    type Output = LinearExpr;
+    fn neg(mut self) -> Self::Output {
+        for c in &mut self.coeffs {
+            *c = -*c;
+        }
+        self.constant = -self.constant;
+        self
+    }
+}
+impl<L: Into<LinearExpr>> std::ops::SubAssign<L> for LinearExpr {
+    fn sub_assign(&mut self, rhs: L) {
+        *self += -rhs.into();
+    }
+}
 impl std::ops::AddAssign<i64> for LinearExpr {
     fn add_assign(&mut self, rhs: i64) {
         self.constant += rhs;
@@ -395,6 +530,16 @@ where
         self
     }
 }
+impl<T> std::ops::Sub<T> for LinearExpr
+where
+    LinearExpr: std::ops::SubAssign<T>,
+{
+    type Output = LinearExpr;
+    fn sub(mut self, rhs: T) -> Self::Output {
+        self -= rhs;
+        self
+    }
+}
 impl From<LinearExpr> for proto::LinearExpressionProto {
     fn from(expr: LinearExpr) -> Self {
         proto::LinearExpressionProto {
@@ -404,27 +549,3 @@ impl From<LinearExpr> for proto::LinearExpressionProto {
         }
     }
 }
-
-fn create_linear_cst_proto(
-    left: &LinearExpr,
-    right: &LinearExpr,
-) -> (proto::LinearConstraintProto, i64) {
-    (
-        proto::LinearConstraintProto {
-            vars: left
-                .vars
-                .iter()
-                .copied()
-                .chain(right.vars.iter().copied())
-                .collect(),
-            coeffs: left
-                .coeffs
-                .iter()
-                .copied()
-                .chain(right.coeffs.iter().map(|&c| -c))
-                .collect(),
-            domain: vec![],
-        },
-        right.constant - left.constant,
-    )
-}