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diff --git a/src/main/java/bjc/optics/LensX.java b/src/main/java/bjc/optics/LensX.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 static bjc.optics.Lenses.immutable;
+
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+/**
+ * A type-variant lens
+ * 
+ * @author Ben Culkin
+ *
+ * @param <W1> The first type the lens is used on
+ * @param <W2> The second 'whole' type
+ * @param <P1> The first 'part' type
+ * @param <P2> The second 'part' type
+ */
+public interface LensX<W1, W2, P1, P2> extends Optic<W1, W2, P1, P2> {
+	/**
+	 * Retrieve the focused value of this lens.
+	 * 
+	 * @param source The item to use the lens on.
+	 * 
+	 * @return The value from the given whole this lens focuses on.
+	 */
+	P1 get(W1 source);
+
+	/**
+	 * Create an updated version of the item this lens focuses on.
+	 * 
+	 * @param source The item to use the lens on.
+	 * @param val    The new value.
+	 * 
+	 * @return The updated item the lens was used on.
+	 */
+	W2 set(W1 source, P2 val);
+
+	/**
+	 * Update the focused value.
+	 * 
+	 * NOTE: It will often be more efficient to implement this directly. The
+	 * implementation here is provided for convenience.
+	 * 
+	 * @param source The item to use the lens on.
+	 * @param action The action to applied to the focused item.
+	 * 
+	 * @return The updated item the lens was used on.
+	 */
+	default W2 update(W1 source, Function<P1, P2> action) {
+		return set(source, action.apply(get(source)));
+	}
+	
+	/**
+	 * Compose two type-variant lenses together.
+	 * 
+	 * @param <V1> The first type the second lens focuses on
+	 * @param <V2> The second type the second lens focuses on.
+	 * 
+	 * @param other The second lens to use.
+	 * 
+	 * @return A lens composed from this one and the given one.
+	 */
+	default <V1, V2> LensX<W1, W2, V1, V2> compose(LensX<P1, P2, V1, V2> other) {
+		return immutable((whole) -> other.get(get(whole)),
+				(whole, val) -> update(whole, (P1 val2) -> other.set(val2, val)));
+	}
+}
+\ No newline at end of file