Index: trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/IMultiObjectiveOperator.cs
===================================================================
--- trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/IMultiObjectiveOperator.cs	(revision 11971)
+++ trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/IMultiObjectiveOperator.cs	(revision 11971)
@@ -0,0 +1,52 @@
+﻿#region License Information
+/* HeuristicLab
+ * Copyright (C) 2002-2015 Heuristic and Evolutionary Algorithms Laboratory (HEAL)
+ *
+ * This file is part of HeuristicLab.
+ *
+ * HeuristicLab is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * HeuristicLab is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+namespace HeuristicLab.Optimization {
+  /// <summary>
+  /// This is only a marker interface to prevent operators that expect to work with multiple quality values
+  /// and/or multiple maximization flags to appear in single-objective algorithms.
+  /// </summary>
+  /// <remarks>
+  /// Marker interfaces are typically not a good design, however specifying a reasonable non-empty common
+  /// interface for all multi-objective operators is difficult. There are many different variants of
+  /// multi-ojective operators with specific needs that can currently only be abstracted by an empty interface.
+  /// In the following, please find a list of different kinds of multi-objective operators.
+  /// <list type="bullet">
+  ///   <item>
+  ///     <term>Evaluators</term>
+  ///     <description>These only need a qualities parameter, but not a maximization parameter.</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Population-based operators</term>
+  ///     <description>These require the qualities parameter to be a scope-tree lookup, e.g. analyzers, crossovers, multi-objective selectors.</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Problem-specific operators</term>
+  ///     <description>They may perform multiple operations on a solution, evaluate and select without taking into account maximization or the evaluator</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Other multi-objective operators</term>
+  ///     <description>They would need both a qualities, and a maximization parameter, and may be population-based or not.</description>
+  ///   </item>
+  /// </list>
+  /// </remarks>
+  public interface IMultiObjectiveOperator { }
+}
Index: trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/ISingleObjectiveOperator.cs
===================================================================
--- trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/ISingleObjectiveOperator.cs	(revision 11971)
+++ trunk/sources/HeuristicLab.Optimization/3.3/Interfaces/ISingleObjectiveOperator.cs	(revision 11971)
@@ -0,0 +1,52 @@
+﻿#region License Information
+/* HeuristicLab
+ * Copyright (C) 2002-2015 Heuristic and Evolutionary Algorithms Laboratory (HEAL)
+ *
+ * This file is part of HeuristicLab.
+ *
+ * HeuristicLab is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * HeuristicLab is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+namespace HeuristicLab.Optimization {
+  /// <summary>
+  /// This is only a marker interface to prevent operators that expect to work with a single quality value
+  /// and/or a single maximization flag to appear in multi-objective algorithms.
+  /// </summary>
+  /// <remarks>
+  /// Marker interfaces are typically not a good design, however specifying a reasonable non-empty common
+  /// interface for all single-objective operators is difficult. There are many different variants of
+  /// single-ojective operators with specific needs that can currently only be abstracted by an empty interface.
+  /// In the following, please find a list of different kinds of single-objective operators.
+  /// <list type="bullet">
+  ///   <item>
+  ///     <term>Evaluators</term>
+  ///     <description>These only need a quality parameter, but not a maximization parameter.</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Population-based operators</term>
+  ///     <description>These require the quality parameter to be a scope-tree lookup, e.g. analyzers, crossovers, single-objective replacers and selectors.</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Problem-specific operators</term>
+  ///     <description>They may perform multiple operations on a solution, evaluate and select without taking into account maximization or the evaluator, e.g. many improvement operators.</description>
+  ///   </item>
+  ///   <item>
+  ///     <term>Other single-objective operators</term>
+  ///     <description>They would need both a quality, and a maximization parameter, and may be population-based or not.</description>
+  ///   </item>
+  /// </list>
+  /// </remarks>
+  public interface ISingleObjectiveOperator { }
+}
