Implement parsing functions.

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

@@ -0,0 +1,47 @@
+package org.nwapw.abacus.tree;
+
+import java.util.ArrayList;
+
+public class FunctionNode extends TreeNode {
+
+    private String function;
+    private ArrayList<TreeNode> children;
+
+    private FunctionNode() { }
+
+    public FunctionNode(String function){
+        this.function = function;
+        children = new ArrayList<>();
+    }
+
+    public String getFunction() {
+        return function;
+    }
+
+    public void addChild(TreeNode node){
+        children.add(node);
+    }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        Object[] reducedChildren = new Object[children.size()];
+        for(int i = 0; i < reducedChildren.length; i++){
+            reducedChildren[i] = children.get(i).reduce(reducer);
+            if(reducedChildren[i] == null) return null;
+        }
+        return reducer.reduceNode(this, reducedChildren);
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder buffer = new StringBuilder();
+        buffer.append(function);
+        buffer.append("(");
+        for(int i = 0; i < children.size(); i++){
+            buffer.append(children.get(i));
+            buffer.append(i == children.size() - 1 ? "" : ", ");
+        }
+        buffer.append(")");
+        return buffer.toString();
+    }
+}