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diff --git a/src/main/java/bjc/optics/MutableLens.java b/src/main/java/bjc/optics/MutableLens.java
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+/* 
+ * esodata - data structures and other things, of varying utility
+ * Copyright 2022, Ben Culkin
+ * 
+ * This program 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.
+ *   
+ * This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package bjc.optics;
+
+/**
+ * A type-invariant lens for mutating objects.
+ * 
+ * Note that there is no type-variant version, because that wouldn't make much sense.
+ * 
+ * Also, mixing mutable and immutable lenses may lead to confusion.
+ * 
+ * @author bjcul
+ *
+ * @param <Whole> The type the lens is used on
+ * @param <Part> The type the lens is focused on
+ */
+public interface MutableLens<Whole, Part> extends Lens<Whole, Part> {
+	/**
+	 * Apply a mutation to an item.
+	 * 
+	 * @param source The item to use the lens on.
+	 * @param val The new value for the focused field.
+	 */
+	void mutate(Whole source, Part val);
+	
+	@Override
+	default Whole set(Whole source, Part val) {
+		mutate(source, val);
+		return source;
+	}
+}