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diff --git a/watchmaker/examples/src/java/main/org/uncommons/watchmaker/examples/monalisa/AbstractVertexMutation.java b/watchmaker/examples/src/java/main/org/uncommons/watchmaker/examples/monalisa/AbstractVertexMutation.java
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+//=============================================================================
+// Copyright 2006-2010 Daniel W. Dyer
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//=============================================================================
+package org.uncommons.watchmaker.examples.monalisa;
+
+import java.awt.Dimension;
+import java.awt.Point;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+import org.uncommons.maths.number.NumberGenerator;
+import org.uncommons.maths.random.Probability;
+import org.uncommons.watchmaker.framework.EvolutionaryOperator;
+
+/**
+ * Base class for mutation operators that modify the points of polygons in an
+ * image.
+ * @author Daniel Dyer
+ */
+abstract class AbstractVertexMutation implements EvolutionaryOperator<ColouredPolygon>
+{
+    private final Dimension canvasSize;
+    private final NumberGenerator<Probability> mutationProbability;
+
+
+    /**
+     * @param mutationProbability A {@link NumberGenerator} that controls the probability
+     * that a polygon's points will be mutated.
+     * @param canvasSize The size of the canvas.  Used to constrain the positions of the points.
+     */
+    protected AbstractVertexMutation(NumberGenerator<Probability> mutationProbability,
+                                     Dimension canvasSize)
+    {
+        this.mutationProbability = mutationProbability;
+        this.canvasSize = canvasSize;
+    }
+
+
+    /**
+     * @return The dimensions of the target image.
+     */
+    protected Dimension getCanvasSize()
+    {
+        return canvasSize;
+    }
+
+
+    /**
+     * @return The {@link NumberGenerator} that provides the mutation probability.
+     */
+    protected NumberGenerator<Probability> getMutationProbability()
+    {
+        return mutationProbability;
+    }
+
+
+    /**
+     * Applies the mutation to each polygon in the list provided according to the
+     * pre-configured mutation probability.  If the probability is 0.1, approximately
+     * 10% of the individuals will be mutated.  The actual mutation operation is
+     * defined in the sub-class implementation of the
+     * {@link #mutateVertices(java.util.List, java.util.Random)} method.
+     * @param polygons The list of polygons to be mutated.
+     * @param rng A source of randomness.
+     * @return The polygons after mutation.  None, some or all will have been
+     * modified. 
+     */
+    public List<ColouredPolygon> apply(List<ColouredPolygon> polygons, Random rng)
+    {
+        List<ColouredPolygon> newPolygons = new ArrayList<ColouredPolygon>(polygons.size());
+        for (ColouredPolygon polygon : polygons)
+        {
+            List<Point> newVertices = mutateVertices(polygon.getVertices(), rng);
+            newPolygons.add(newVertices == polygon.getVertices()
+                            ? polygon
+                            : new ColouredPolygon(polygon.getColour(), newVertices));
+        }
+        return newPolygons;
+    }
+
+
+    /**
+     * Implemented in sub-classes to perform the mutation of the vertices.
+     * @param vertices A list of the points that make up the polygon.
+     * @param rng A source of randomness.
+     * @return A mutated list of points.
+     */
+    protected abstract List<Point> mutateVertices(List<Point> vertices, Random rng);
+}