Implement correct plugin loading and registration.

src/org/nwapw/abacus/tree/TreeNode.java

@@ -11,6 +11,12 @@ import java.util.*;
  */
 public abstract class TreeNode {
 
+    /**
+     * The function that reduces a tree to a single vale.
+     * @param reducer the reducer used to reduce the tree.
+     * @param <T> the type the reducer produces.
+     * @return the result of the reduction, or null on error.
+     */
     public abstract <T> T reduce(Reducer<T> reducer);
 
 }