/**
* Specifies the constant modes (near null space) of the matrix. This
* parameter tells AMG whether we work on a scalar equation (where the
- * near null space only consists of ones) or on a vector-valued
- * equation.
+ * near null space only consists of ones, and default value is OK) or on
+ * a vector-valued equation.
+ * For vector-valued equation problem with <tt>n_component</tt>, the
+ * provided @p constant_modes should fulfill the following requirements:
+ * <ul>
+ * <li> n_component.size() == <tt>n_component</tt> </li>
+ * <li> n_component[*].size() == n_dof_local or
+ * n_component[*].size() == n_dof_global </li>
+ * <li> n_component[<tt>ic</tt>][<tt>id</tt>] == "<tt>id</tt><em>th</em>
+ * DoF is corresponding to component <tt>ic</tt> </li>
+ * </ul>
*/
std::vector<std::vector<bool> > constant_modes;