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diff --git a/src/main/java/com/google/escapevelocity/Node.java b/src/main/java/com/google/escapevelocity/Node.java
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+/*
+ * Copyright (C) 2018 Google, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.escapevelocity;
+
+import com.google.common.collect.ImmutableList;
+
+/**
+ * A node in the parse tree.
+ *
+ * @author emcmanus@google.com (Éamonn McManus)
+ */
+abstract class Node {
+  final String resourceName;
+  final int lineNumber;
+
+  Node(String resourceName, int lineNumber) {
+    this.resourceName = resourceName;
+    this.lineNumber = lineNumber;
+  }
+
+  /**
+   * Returns the result of evaluating this node in the given context. This result may be used as
+   * part of a further operation, for example evaluating {@code 2 + 3} to 5 in order to set
+   * {@code $x} to 5 in {@code #set ($x = 2 + 3)}. Or it may be used directly as part of the
+   * template output, for example evaluating replacing {@code name} by {@code Fred} in
+   * {@code My name is $name.}.
+   */
+  abstract Object evaluate(EvaluationContext context);
+
+  private String where() {
+    String where = "In expression on line " + lineNumber;
+    if (resourceName != null) {
+      where += " of " + resourceName;
+    }
+    return where;
+  }
+
+  EvaluationException evaluationException(String message) {
+    return new EvaluationException(where() + ": " + message);
+  }
+
+  EvaluationException evaluationException(Throwable cause) {
+    return new EvaluationException(where() + ": " + cause, cause);
+  }
+
+  /**
+   * Returns an empty node in the parse tree. This is used for example to represent the trivial
+   * "else" part of an {@code #if} that does not have an explicit {@code #else}.
+   */
+  static Node emptyNode(String resourceName, int lineNumber) {
+    return new Cons(resourceName, lineNumber, ImmutableList.<Node>of());
+  }
+
+  /**
+   * Create a new parse tree node that is the concatenation of the given ones. Evaluating the
+   * new node produces the same string as evaluating each of the given nodes and concatenating the
+   * result.
+   */
+  static Node cons(String resourceName, int lineNumber, ImmutableList<Node> nodes) {
+    return new Cons(resourceName, lineNumber, nodes);
+  }
+
+  private static final class Cons extends Node {
+    private final ImmutableList<Node> nodes;
+
+    Cons(String resourceName, int lineNumber, ImmutableList<Node> nodes) {
+      super(resourceName, lineNumber);
+      this.nodes = nodes;
+    }
+
+    @Override Object evaluate(EvaluationContext context) {
+      StringBuilder sb = new StringBuilder();
+      for (Node node : nodes) {
+        sb.append(node.evaluate(context));
+      }
+      return sb.toString();
+    }
+  }
+}