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diff --git a/base/src/main/java/bjc/utils/data/ITree.java b/base/src/main/java/bjc/utils/data/ITree.java
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+++ b/base/src/main/java/bjc/utils/data/ITree.java
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+package bjc.utils.data;
+
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.function.UnaryOperator;
+
+import bjc.utils.funcdata.bst.TreeLinearizationMethod;
+import bjc.utils.functypes.ListFlattener;
+
+/**
+ * A node in a homogeneous tree with a unlimited amount of children.
+ *
+ * @author ben
+ *
+ * @param <ContainedType>
+ *                The type of data contained in the tree nodes.
+ *
+ */
+public interface ITree<ContainedType> {
+	/**
+	 * Append a child to this node.
+	 *
+	 * @param child
+	 *                The child to append to this node.
+	 */
+	void addChild(ITree<ContainedType> child);
+
+	/**
+	 * Prepend a child to this node.
+	 *
+	 * @param child
+	 *                The child to prepend to this node.
+	 */
+	void prependChild(ITree<ContainedType> child);
+
+	/**
+	 * Collapse a tree into a single version.
+	 *
+	 * @param <NewType>
+	 *                The intermediate type being folded.
+	 *
+	 * @param <ReturnedType>
+	 *                The type that is the end result.
+	 *
+	 * @param leafTransform
+	 *                The function to use to convert leaf values.
+	 *
+	 * @param nodeCollapser
+	 *                The function to use to convert internal nodes and
+	 *                their children.
+	 *
+	 * @param resultTransformer
+	 *                The function to use to convert a state to the returned
+	 *                version.
+	 *
+	 * @return The final transformed state.
+	 */
+	<NewType, ReturnedType> ReturnedType collapse(Function<ContainedType, NewType> leafTransform,
+			Function<ContainedType, ListFlattener<NewType>> nodeCollapser,
+			Function<NewType, ReturnedType> resultTransformer);
+
+	/**
+	 * Execute a given action for each of this tree's children.
+	 *
+	 * @param action
+	 *                The action to execute for each child.
+	 */
+	void doForChildren(Consumer<ITree<ContainedType>> action);
+
+	/**
+	 * Expand the nodes of a tree into trees, and then merge the contents of
+	 * those trees into a single tree.
+	 *
+	 * @param mapper
+	 *                The function to use to map values into trees.
+	 *
+	 * @return A tree, with some nodes expanded into trees.
+	 */
+	default ITree<ContainedType> flatMapTree(final Function<ContainedType, ITree<ContainedType>> mapper) {
+		return topDownTransform(dat -> TopDownTransformResult.PUSHDOWN, node -> {
+			if (node.getChildrenCount() > 0) {
+				final ITree<ContainedType> parent = node.transformHead(mapper);
+
+				node.doForChildren(parent::addChild);
+
+				return parent;
+			}
+
+			return node.transformHead(mapper);
+		});
+	}
+
+	/**
+	 * Get the specified child of this tree.
+	 *
+	 * @param childNo
+	 *                The number of the child to get.
+	 *
+	 * @return The specified child of this tree.
+	 */
+	default ITree<ContainedType> getChild(final int childNo) {
+		return transformChild(childNo, child -> child);
+	}
+
+	/**
+	 * Get a count of the number of direct children this node has.
+	 *
+	 * @return The number of direct children this node has.
+	 */
+	int getChildrenCount();
+
+	/**
+	 * Get the data stored in this node.
+	 *
+	 * @return The data stored in this node.
+	 */
+	default ContainedType getHead() {
+		return transformHead(head -> head);
+	}
+
+	/**
+	 * Rebuild the tree with the same structure, but different nodes.
+	 *
+	 * @param <MappedType>
+	 *                The type of the new tree.
+	 *
+	 * @param leafTransformer
+	 *                The function to use to transform leaf tokens.
+	 *
+	 * @param operatorTransformer
+	 *                The function to use to transform internal tokens.
+	 *
+	 * @return The tree, with the nodes changed.
+	 */
+	<MappedType> ITree<MappedType> rebuildTree(Function<ContainedType, MappedType> leafTransformer,
+			Function<ContainedType, MappedType> operatorTransformer);
+
+	/**
+	 * Transform some of the nodes in this tree.
+	 *
+	 * @param nodePicker
+	 *                The predicate to use to pick nodes to transform.
+	 *
+	 * @param transformer
+	 *                The function to use to transform picked nodes.
+	 */
+	void selectiveTransform(Predicate<ContainedType> nodePicker, UnaryOperator<ContainedType> transformer);
+
+	/**
+	 * Do a top-down transform of the tree.
+	 *
+	 * @param transformPicker
+	 *                The function to use to pick how to progress.
+	 *
+	 * @param transformer
+	 *                The function used to transform picked subtrees.
+	 *
+	 * @return The tree with the transform applied to picked subtrees.
+	 */
+	ITree<ContainedType> topDownTransform(Function<ContainedType, TopDownTransformResult> transformPicker,
+			UnaryOperator<ITree<ContainedType>> transformer);
+
+	/**
+	 * Transform one of this nodes children.
+	 *
+	 * @param <TransformedType>
+	 *                The type of the transformed value.
+	 *
+	 * @param childNo
+	 *                The number of the child to transform.
+	 *
+	 * @param transformer
+	 *                The function to use to transform the value.
+	 *
+	 * @return The transformed value.
+	 *
+	 * @throws IllegalArgumentException
+	 *                 if the childNo is out of bounds (0 <= childNo <=
+	 *                 childCount()).
+	 */
+	<TransformedType> TransformedType transformChild(int childNo,
+			Function<ITree<ContainedType>, TransformedType> transformer);
+
+	/**
+	 * Transform the value that is the head of this node.
+	 *
+	 * @param <TransformedType>
+	 *                The type of the transformed value.
+	 *
+	 * @param transformer
+	 *                The function to use to transform the value.
+	 *
+	 * @return The transformed value.
+	 */
+	<TransformedType> TransformedType transformHead(Function<ContainedType, TransformedType> transformer);
+
+	/**
+	 * Transform the tree into a tree with a different type of token.
+	 *
+	 * @param <MappedType>
+	 *                The type of the new tree.
+	 *
+	 * @param transformer
+	 *                The function to use to transform tokens.
+	 *
+	 * @return A tree with the token types transformed.
+	 */
+	default <MappedType> ITree<MappedType> transformTree(final Function<ContainedType, MappedType> transformer) {
+		return rebuildTree(transformer, transformer);
+	}
+
+	/**
+	 * Perform an action on each part of the tree.
+	 *
+	 * @param linearizationMethod
+	 *                The way to traverse the tree.
+	 *
+	 * @param action
+	 *                The action to perform on each tree node.
+	 */
+	void traverse(TreeLinearizationMethod linearizationMethod, Consumer<ContainedType> action);
+
+	/**
+	 * Find the farthest to right child that satisfies the given predicate.
+	 *
+	 * @param childPred
+	 *                The predicate to satisfy.
+	 *
+	 * @return The index of the right-most child that satisfies the
+	 *         predicate, or -1 if one doesn't exist.
+	 */
+	int revFind(Predicate<ITree<ContainedType>> childPred);
+}