diff --git a/README.md b/README.md
index f4db49e..7030965 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,7 @@ The physics is coded in C++, with Python as user interface for easy and fast usa
 - [ ] Write full benchmark
   - [ ] In C++.
   - [ ] In Python.
+- [ ] Mesh refiner, rings and sectors for cell
 - [ ] Enable users to test the lib with codespace and notebooks.
 - [ ] GPU offload.
 
diff --git a/examples/main.cpp b/examples/main.cpp
index f8870b7..e643a1b 100644
--- a/examples/main.cpp
+++ b/examples/main.cpp
@@ -10,6 +10,7 @@ int main() {
 
   CheckOpenMP();
 
+  // Define materials
   ArrayXd sigma_t{{0.222222, 0.833333}};
   ArrayXXd sigma_s{{0.00, 0.02}, {0.00, 0.00}};
   ArrayXd sigma_a{{0.010120, 0.080032}};
@@ -34,6 +35,7 @@ int main() {
   for (const auto* m : {&UO2, &M4, &H2O}) std::cout << *m << '\n';
   std::cout << '\n';
 
+  // Define cells
   double side = 1.0;
   Cell UO2cell(side, {0.5}, {UO2, H2O}, "UO2");
   Cell M4cell(side, {0.4, 0.6}, {M4, UO2, H2O}, "M4");
@@ -42,13 +44,26 @@ int main() {
   for (const auto* c : {&UO2cell, &M4cell, &H2Ocell}) std::cout << *c << '\n';
   std::cout << '\n';
 
+  // Define assemblies
   Lattice assm00{{UO2cell, H2Ocell, UO2cell, H2Ocell}, "assm00"};
   Lattice assm01{{M4cell, UO2cell, H2Ocell, H2Ocell}, "assm01"};
   Lattice assm10{{UO2cell, H2Ocell, UO2cell, M4cell}, "assm10"};
   Lattice assm11{{UO2cell, M4cell, H2Ocell, UO2cell}, "assm11"};
 
+  // Define core
   Lattice core{{assm00, assm01, assm10, assm11}, "core"};
   std::cout << core << '\n';
 
+  // Set boundary conditions
+  // core.BoundaryCondition(BoundarySide::xmin, BoundaryCondition::Vacuum);
+  // core.BoundaryCondition(BoundarySide::xmax, BoundaryCondition::Reflection);
+
+  // Define solver and solve the transport equation
+  // SNConfig sn(2); // SN type, order
+  // FEMConfig dg(2); // fem type, order
+  // Solver solver(sn, dg);
+  // solver.geometry(core);
+  // solver.solve();
+
   return 0;
 }
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diff --git a/src/demeter/model.hpp b/src/demeter/model.hpp
index fe31217..52fda22 100644
--- a/src/demeter/model.hpp
+++ b/src/demeter/model.hpp
@@ -2,4 +2,5 @@
 
 #include "model/cell.hpp"
 #include "model/material.hpp"
-#include "model/lattice.hpp"
\ No newline at end of file
+#include "model/lattice.hpp"
+#include "model/boundary.hpp"
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diff --git a/src/demeter/model/boundary.hpp b/src/demeter/model/boundary.hpp
new file mode 100644
index 0000000..8769305
--- /dev/null
+++ b/src/demeter/model/boundary.hpp
@@ -0,0 +1,25 @@
+#pragma once
+
+#include "demeter/common.hpp"
+#include "demeter/model/lattice.hpp"
+
+namespace Demeter {
+
+/**
+ * @brief Possible boundary condition
+ */
+enum class BoundaryCondition { Vacuum, Reflection };
+
+/**
+ * @brief Possible side of a lattice
+ */
+enum class BoundarySide {
+  xmin,
+  xmax,
+  ymin,
+  ymax,
+  zmin,
+  zmax
+};  // TODO diagonal
+
+}  // namespace Demeter
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diff --git a/src/demeter/solve/solver.hpp b/src/demeter/solve/solver.hpp
new file mode 100644
index 0000000..ad3d651
--- /dev/null
+++ b/src/demeter/solve/solver.hpp
@@ -0,0 +1,12 @@
+#pragma once
+
+#include "demeter/common.hpp"
+#include "demeter/model/lattice.hpp"
+
+namespace Demeter {
+
+class Solver {
+ public:
+
+};
+}  // namespace Demeter
\ No newline at end of file