Move the source files into a new default directory.

src/main/java/org/nwapw/abacus/tree/Reducer.java

@@ -0,0 +1,17 @@
+package org.nwapw.abacus.tree;
+
+/**
+ * Interface used to reduce a tree into a single value.
+ * @param <T> the value to reduce into.
+ */
+public interface Reducer<T> {
+
+    /**
+     * Reduces the given tree into a single value of type T.
+     * @param node the node being passed in to be reduced.
+     * @param children the already-reduced children of this node.
+     * @return the resulting value from the reduce.
+     */
+    public T reduceNode(TreeNode node, Object...children);
+
+}