Fix function argument order.

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

@@ -39,13 +39,21 @@ public class FunctionNode extends TreeNode {
     }
 
     /**
-     * Adds a child to this node.
+     * Adds a child to the end of this node's child list.
      * @param node the child to add.
      */
-    public void addChild(TreeNode node){
+    public void appendChild(TreeNode node){
         children.add(node);
     }
 
+    /**
+     * Adds a new child to the beginning of this node's child list.
+     * @param node the node to add.
+     */
+    public void prependChild(TreeNode node) {
+        children.add(0, node);
+    }
+
     @Override
     public <T> T reduce(Reducer<T> reducer) {
         Object[] reducedChildren = new Object[children.size()];