Add the withReducer variants of the Applier functions.

core/src/main/java/org/nwapw/abacus/function/TreeValueFunction.java

@@ -1,6 +1,7 @@
 package org.nwapw.abacus.function;
 
 import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.tree.Reducer;
 import org.nwapw.abacus.tree.TreeNode;
 
 /**
@@ -9,4 +10,34 @@ import org.nwapw.abacus.tree.TreeNode;
  */
 public abstract class TreeValueFunction extends Applicable<TreeNode, NumberInterface> {
 
+    @Override
+    protected NumberInterface applyInternal(TreeNode[] params) {
+        return null;
+    }
+
+    @Override
+    public NumberInterface apply(TreeNode... params) {
+        return null;
+    }
+
+    /**
+     * Applies the tree value functions to the given tree nodes,
+     * using the given reducer.
+     * @param reducer the reducer to use.
+     * @param params the parameters to apply to.
+     * @return the result of the application.
+     */
+    public abstract NumberInterface applyWithReducerInternal(Reducer<NumberInterface> reducer, TreeNode[] params);
+
+    /**
+     * Checks if the given parameters and reducer can be used
+     * with this function, and if so, calls the function on them.
+     * @param reducer the reducer to use.
+     * @param params the parameters to apply to.
+     * @return the result of the application, or null if the parameters are incompatible.
+     */
+    public NumberInterface applyWithReducer(Reducer<NumberInterface> reducer, TreeNode... params) {
+        if(!matchesParams(params) || reducer == null) return null;
+        return applyWithReducerInternal(reducer, params);
+    }
 }