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diff --git a/src/main/java/bjc/optics/Lens.java b/src/main/java/bjc/optics/Lens.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;
+
+import java.util.function.Function;
+import java.util.function.UnaryOperator;
+
+import bjc.typeclasses.BiContainer;
+
+/**
+ * A type-invariant var Laarhoven lens.
+ * 
+ * @author bjcul
+ *
+ * @param <Whole> The item this lens can focus on
+ * @param <Part> The field this lens focuses on
+ */
+public interface Lens<Whole, Part> extends LensX<Whole, Whole, Part, Part>, BiContainer<Whole, Part, Lens<Whole, Part>> {
+	/**
+	 * Modify a given whole using an operation
+	 * 
+	 * @param source The whole to modify.
+	 * @param mod The operation to use for modifying a part
+	 * 
+	 * @return A modified whole
+	 */
+	default Whole modify(Whole source, UnaryOperator<Part> mod) {
+		return set(source, mod.apply(get(source)));
+	}
+	
+	/**
+	 * Create a function which sets the part of a given whole.
+	 * 
+	 * @param part The part to set
+	 * 
+	 * @return A function that sets the given part on a whole.
+	 */
+	default Function<Whole, Whole> setting(Part part) {
+		return (whole) -> set(whole, part);
+	}
+	
+	/**
+	 * Lift a function that modifies parts to one that modifies wholes.
+	 * 
+	 * @param f The function which operates on parts
+	 * 
+	 * @return A corresponding function which applies the given modification to a part.
+	 */
+	default Function<Whole, Whole> lift(UnaryOperator<Part> f) {
+		// modify will be more efficient for some lenses
+		return (whole) -> modify(whole, f);
+	}
+}