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diff --git a/src/main/java/bjc/funcdata/bst/TreePart.java b/src/main/java/bjc/funcdata/bst/TreePart.java
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+++ b/src/main/java/bjc/funcdata/bst/TreePart.java
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+package bjc.funcdata.bst;
+
+import java.util.Comparator;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+/**
+ * A interface for the fundamental things that want to be part of a tree.
+ *
+ * @author ben
+ *
+ * @param <T>
+ *            The data contained in this part of the tree.
+ */
+public interface TreePart<T> {
+	/**
+	 * Add a element below this tree part somewhere.
+	 *
+	 * @param element
+	 *                   The element to add below this tree part
+	 *
+	 * @param comparator
+	 *                   The thing to use for comparing values to find where to
+	 *                   insert the tree part.
+	 */
+	public void add(T element, Comparator<T> comparator);
+
+	/**
+	 * Collapses this tree part into a single value.
+	 *
+	 * Does not change the underlying tree.
+	 *
+	 * @param <E>
+	 *                        The type of the final collapsed value
+	 *
+	 * @param nodeCollapser
+	 *                        The function to use to transform data into mapped
+	 *                        form.
+	 *
+	 * @param branchCollapser
+	 *                        The function to use to collapse data in mapped form
+	 *                        into a single value.
+	 *
+	 * @return A single value from collapsing the tree.
+	 */
+	public <E> E collapse(Function<T, E> nodeCollapser,
+			BiFunction<E, E, E> branchCollapser);
+
+	/**
+	 * Check if this tre part or below it contains the specified data item.
+	 *
+	 * @param element
+	 *                   The data item to look for.
+	 *
+	 * @param comparator
+	 *                   The comparator to use to search for the data item.
+	 *
+	 * @return Whether or not the given item is contained in this tree part or its
+	 *         children.
+	 */
+	public boolean contains(T element, Comparator<T> comparator);
+
+	/**
+	 * Get the data associated with this tree part.
+	 *
+	 * @return The data associated with this tree part.
+	 */
+	public T data();
+
+	/**
+	 * Remove the given node from this tree part and any of its children.
+	 *
+	 * @param element
+	 *                   The data item to remove.
+	 *
+	 * @param comparator
+	 *                   The comparator to use to search for the data item.
+	 */
+	public void delete(T element, Comparator<T> comparator);
+
+	/**
+	 * Execute a directed walk through the tree.
+	 *
+	 * @param walker
+	 *               The function to use to direct the walk through the tree.
+	 *
+	 * @return Whether the directed walk finished successfully.
+	 */
+	public boolean directedWalk(DirectedWalkFunction<T> walker);
+
+	/**
+	 * Execute a provided function for each element of tree it succesfully completes
+	 * for.
+	 *
+	 * @param linearizationMethod
+	 *                            The way to linearize the tree for executing.
+	 *
+	 * @param predicate
+	 *                            The predicate to apply to each element, where it
+	 *                            returning false terminates traversal early.
+	 *
+	 * @return Whether the traversal finished succesfully.
+	 */
+	public boolean forEach(TreeLinearizationMethod linearizationMethod,
+			Predicate<T> predicate);
+}