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diff --git a/eigen/doc/examples/matrixfree_cg.cpp b/eigen/doc/examples/matrixfree_cg.cpp
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+++ b/eigen/doc/examples/matrixfree_cg.cpp
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+#include <iostream>
+#include <Eigen/Core>
+#include <Eigen/Dense>
+#include <Eigen/IterativeLinearSolvers>
+
+class MatrixReplacement;
+template<typename Rhs> class MatrixReplacement_ProductReturnType;
+
+namespace Eigen {
+namespace internal {
+  template<>
+  struct traits<MatrixReplacement> :  Eigen::internal::traits<Eigen::SparseMatrix<double> >
+  {};
+
+  template <typename Rhs>
+  struct traits<MatrixReplacement_ProductReturnType<Rhs> > {
+    // The equivalent plain objet type of the product. This type is used if the product needs to be evaluated into a temporary.
+    typedef Eigen::Matrix<typename Rhs::Scalar, Eigen::Dynamic, Rhs::ColsAtCompileTime> ReturnType;
+  };
+}
+}
+
+// Inheriting EigenBase should not be needed in the future.
+class MatrixReplacement : public Eigen::EigenBase<MatrixReplacement> {
+public:
+  // Expose some compile-time information to Eigen:
+  typedef double Scalar;
+  typedef double RealScalar;
+  enum {
+    ColsAtCompileTime = Eigen::Dynamic,
+    RowsAtCompileTime = Eigen::Dynamic,
+    MaxColsAtCompileTime = Eigen::Dynamic,
+    MaxRowsAtCompileTime = Eigen::Dynamic
+  };
+
+  Index rows() const { return 4; }
+  Index cols() const { return 4; }
+
+  void resize(Index a_rows, Index a_cols)
+  {
+    // This method should not be needed in the future.
+    assert(a_rows==0 && a_cols==0 || a_rows==rows() && a_cols==cols());
+  }
+
+  // In the future, the return type should be Eigen::Product<MatrixReplacement,Rhs>
+  template<typename Rhs>
+  MatrixReplacement_ProductReturnType<Rhs> operator*(const Eigen::MatrixBase<Rhs>& x) const {
+    return MatrixReplacement_ProductReturnType<Rhs>(*this, x.derived());
+  }
+
+};
+
+// The proxy class representing the product of a MatrixReplacement with a MatrixBase<>
+template<typename Rhs>
+class MatrixReplacement_ProductReturnType : public Eigen::ReturnByValue<MatrixReplacement_ProductReturnType<Rhs> > {
+public:
+  typedef MatrixReplacement::Index Index;
+  
+  // The ctor store references to the matrix and right-hand-side object (usually a vector).
+  MatrixReplacement_ProductReturnType(const MatrixReplacement& matrix, const Rhs& rhs)
+    : m_matrix(matrix), m_rhs(rhs)
+  {}
+  
+  Index rows() const { return m_matrix.rows(); }
+  Index cols() const { return m_rhs.cols(); }
+
+  // This function is automatically called by Eigen. It must evaluate the product of matrix * rhs into y.
+  template<typename Dest>
+  void evalTo(Dest& y) const
+  {
+    y.setZero(4);
+
+    y(0) += 2 * m_rhs(0); y(1) += 1 * m_rhs(0);
+    y(0) += 1 * m_rhs(1); y(1) += 2 * m_rhs(1); y(2) += 1 * m_rhs(1);
+    y(1) += 1 * m_rhs(2); y(2) += 2 * m_rhs(2); y(3) += 1 * m_rhs(2);
+    y(2) += 1 * m_rhs(3); y(3) += 2 * m_rhs(3);
+  }
+
+protected:
+  const MatrixReplacement& m_matrix;
+  typename Rhs::Nested m_rhs;
+};
+
+
+/*****/
+
+// This class simply warp a diagonal matrix as a Jacobi preconditioner.
+// In the future such simple and generic wrapper should be shipped within Eigen itsel.
+template <typename _Scalar>
+class MyJacobiPreconditioner
+{
+    typedef _Scalar Scalar;
+    typedef Eigen::Matrix<Scalar,Eigen::Dynamic,1> Vector;
+    typedef typename Vector::Index Index;
+
+  public:
+    // this typedef is only to export the scalar type and compile-time dimensions to solve_retval
+    typedef Eigen::Matrix<Scalar,Eigen::Dynamic,Eigen::Dynamic> MatrixType;
+
+    MyJacobiPreconditioner() : m_isInitialized(false) {}
+
+    void setInvDiag(const Eigen::VectorXd &invdiag) {
+      m_invdiag=invdiag;
+      m_isInitialized=true;
+    }
+
+    Index rows() const { return m_invdiag.size(); }
+    Index cols() const { return m_invdiag.size(); }
+    
+    template<typename MatType>
+    MyJacobiPreconditioner& analyzePattern(const MatType& ) { return *this; }
+    
+    template<typename MatType>
+    MyJacobiPreconditioner& factorize(const MatType& mat) { return *this; }
+    
+    template<typename MatType>
+    MyJacobiPreconditioner& compute(const MatType& mat) { return *this; }
+
+    template<typename Rhs, typename Dest>
+    void _solve(const Rhs& b, Dest& x) const
+    {
+      x = m_invdiag.array() * b.array() ;
+    }
+
+    template<typename Rhs> inline const Eigen::internal::solve_retval<MyJacobiPreconditioner, Rhs>
+    solve(const Eigen::MatrixBase<Rhs>& b) const
+    {
+      eigen_assert(m_isInitialized && "MyJacobiPreconditioner is not initialized.");
+      eigen_assert(m_invdiag.size()==b.rows()
+                && "MyJacobiPreconditioner::solve(): invalid number of rows of the right hand side matrix b");
+      return Eigen::internal::solve_retval<MyJacobiPreconditioner, Rhs>(*this, b.derived());
+    }
+
+  protected:
+    Vector m_invdiag;
+    bool m_isInitialized;
+};
+
+namespace Eigen {
+namespace internal {
+
+template<typename _MatrixType, typename Rhs>
+struct solve_retval<MyJacobiPreconditioner<_MatrixType>, Rhs>
+  : solve_retval_base<MyJacobiPreconditioner<_MatrixType>, Rhs>
+{
+  typedef MyJacobiPreconditioner<_MatrixType> Dec;
+  EIGEN_MAKE_SOLVE_HELPERS(Dec,Rhs)
+
+  template<typename Dest> void evalTo(Dest& dst) const
+  {
+    dec()._solve(rhs(),dst);
+  }
+};
+
+}
+}
+
+
+/*****/
+
+
+int main()
+{
+  MatrixReplacement A;
+  Eigen::VectorXd b(4), x;
+  b << 1, 1, 1, 1;
+
+  // solve Ax = b using CG with matrix-free version:
+  Eigen::ConjugateGradient < MatrixReplacement, Eigen::Lower|Eigen::Upper, MyJacobiPreconditioner<double> > cg;
+
+  Eigen::VectorXd invdiag(4);
+  invdiag << 1./3., 1./4., 1./4., 1./3.;
+
+  cg.preconditioner().setInvDiag(invdiag);
+  cg.compute(A);
+  x = cg.solve(b);
+
+  std::cout << "#iterations: " << cg.iterations() << std::endl;
+  std::cout << "estimated error: " << cg.error() << std::endl;
+}