diff --git a/miplearn/components/lazy/mem.py b/miplearn/components/lazy/mem.py
index eb3fc63..a08d2d5 100644
--- a/miplearn/components/lazy/mem.py
+++ b/miplearn/components/lazy/mem.py
@@ -9,16 +9,10 @@ from sklearn.preprocessing import MultiLabelBinarizer
 
 from miplearn.extractors.abstract import FeaturesExtractor
 from miplearn.h5 import H5File
-from miplearn.solvers.gurobi import GurobiModel
+from miplearn.solvers.abstract import AbstractModel
 
 logger = logging.getLogger(__name__)
 
-# TODO: Replace GurobiModel by AbstractModel
-# TODO: fix_violations: remove model.inner
-# TODO: fix_violations: remove `where`
-# TODO: Write documentation
-# TODO: Implement ExpertLazyConstrComponent
-
 
 class MemorizingLazyConstrComponent:
     def __init__(self, clf: Any, extractor: FeaturesExtractor) -> None:
@@ -79,13 +73,14 @@ class MemorizingLazyConstrComponent:
     def before_mip(
         self,
         test_h5: str,
-        model: GurobiModel,
+        model: AbstractModel,
         stats: Dict[str, Any],
     ) -> None:
-        assert self.constrs_ is not None
         if model.lazy_enforce is None:
             return
 
+        assert self.constrs_ is not None
+
         # Read features
         with H5File(test_h5, "r") as h5:
             x_sample = self.extractor.get_instance_features(h5)
@@ -101,5 +96,5 @@ class MemorizingLazyConstrComponent:
         # Enforce constraints
         violations = [self.constrs_[i] for (i, yi) in enumerate(y) if yi > 0.5]
         logger.info(f"Enforcing {len(violations)} constraints ahead-of-time...")
-        model.lazy_enforce(model, violations, "aot")
+        model.lazy_enforce(model, violations)
         stats["Lazy Constraints: AOT"] = len(violations)
diff --git a/miplearn/problems/tsp.py b/miplearn/problems/tsp.py
index c11b057..dd910f4 100644
--- a/miplearn/problems/tsp.py
+++ b/miplearn/problems/tsp.py
@@ -159,13 +159,11 @@ def build_tsp_model(data: Union[str, TravelingSalesmanData]) -> GurobiModel:
                 violations.append(cut_edges)
         return violations
 
-    def lazy_enforce(model: GurobiModel, violations: List[Any], where: str) -> None:
+    def lazy_enforce(model: GurobiModel, violations: List[Any]) -> None:
         for violation in violations:
-            constr = quicksum(model.inner._x[e[0], e[1]] for e in violation) >= 2
-            if where == "cb":
-                model.inner.cbLazy(constr)
-            else:
-                model.inner.addConstr(constr)
+            model.add_constr(
+                quicksum(model.inner._x[e[0], e[1]] for e in violation) >= 2
+            )
         logger.info(f"tsp: added {len(violations)} subtour elimination constraints")
 
     model.update()
diff --git a/miplearn/solvers/abstract.py b/miplearn/solvers/abstract.py
index f37da41..cf985f7 100644
--- a/miplearn/solvers/abstract.py
+++ b/miplearn/solvers/abstract.py
@@ -3,7 +3,7 @@
 #  Released under the modified BSD license. See COPYING.md for more details.
 
 from abc import ABC, abstractmethod
-from typing import Optional, Dict
+from typing import Optional, Dict, Callable
 
 import numpy as np
 
@@ -16,6 +16,10 @@ class AbstractModel(ABC):
     _supports_node_count = False
     _supports_solution_pool = False
 
+    def __init__(self) -> None:
+        self.lazy_enforce: Optional[Callable] = None
+        self.lazy_separate: Optional[Callable] = None
+
     @abstractmethod
     def add_constrs(
         self,
diff --git a/miplearn/solvers/gurobi.py b/miplearn/solvers/gurobi.py
index 90e1a20..63917d9 100644
--- a/miplearn/solvers/gurobi.py
+++ b/miplearn/solvers/gurobi.py
@@ -28,6 +28,7 @@ class GurobiModel(AbstractModel):
         self.lazy_enforce = lazy_enforce
         self.inner = inner
         self.lazy_constrs_: Optional[List[Any]] = None
+        self.where = "default"
 
     def add_constrs(
         self,
@@ -53,6 +54,14 @@ class GurobiModel(AbstractModel):
                 stats["Added constraints"] = 0
             stats["Added constraints"] += nconstrs
 
+    def add_constr(self, constr: Any) -> None:
+        if self.where == "lazy":
+            self.inner.cbLazy(constr)
+        elif self.where == "cut":
+            self.inner.cbCut(constr)
+        else:
+            self.inner.addConstr(constr)
+
     def extract_after_load(self, h5: H5File) -> None:
         """
         Given a model that has just been loaded, extracts static problem
@@ -132,9 +141,11 @@ class GurobiModel(AbstractModel):
             assert self.lazy_constrs_ is not None
             assert self.lazy_enforce is not None
             if where == GRB.Callback.MIPSOL:
+                self.where = "lazy"
                 violations = self.lazy_separate(self)
                 self.lazy_constrs_.extend(violations)
-                self.lazy_enforce(self, violations, "cb")
+                self.lazy_enforce(self, violations)
+            self.where = "default"
 
         if self.lazy_enforce is not None:
             self.inner.Params.lazyConstraints = 1
diff --git a/tests/fixtures/tsp-n20-00000.h5 b/tests/fixtures/tsp-n20-00000.h5
index d6bf57a..159a981 100644
Binary files a/tests/fixtures/tsp-n20-00000.h5 and b/tests/fixtures/tsp-n20-00000.h5 differ