Add a new constructor to the UI, and move strings into constants.

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

@@ -0,0 +1,26 @@
+package org.nwapw.abacus.tree;
+
+import org.nwapw.abacus.number.NumberInterface;
+import org.nwapw.abacus.plugin.PluginManager;
+
+public class NumberReducer implements Reducer<NumberInterface> {
+
+    private PluginManager manager;
+
+    public NumberReducer(PluginManager manager){
+        this.manager = manager;
+    }
+
+    @Override
+    public NumberInterface reduceNode(TreeNode node, Object... children) {
+        if(node instanceof NumberNode) {
+            return ((NumberNode) node).getNumber();
+        } else if(node instanceof OpNode){
+            NumberInterface left = (NumberInterface) children[0];
+            NumberInterface right = (NumberInterface) children[1];
+            return manager.functionFor(((OpNode) node).getOperation()).apply(left, right);
+        }
+        return null;
+    }
+
+}