Initial commit

Eigen3/doc/snippets/MatrixBase_template_int_start.cpp

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+RowVector4i v = RowVector4i::Random();
+cout << "Here is the vector v:" << endl << v << endl;
+cout << "Here is v.head(2):" << endl << v.head<2>() << endl;
+v.head<2>().setZero();
+cout << "Now the vector v is:" << endl << v << endl;