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diff --git a/src/main/java/com/google/escapevelocity/EvaluationContext.java b/src/main/java/com/google/escapevelocity/EvaluationContext.java
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+++ b/src/main/java/com/google/escapevelocity/EvaluationContext.java
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+/*
+ * Copyright (C) 2015 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 java.util.Map;
+import java.util.TreeMap;
+
+/**
+ * The context of a template evaluation. This consists of the template variables and the template
+ * macros. The template variables start with the values supplied by the evaluation call, and can
+ * be changed by {@code #set} directives and during the execution of {@code #foreach} and macro
+ * calls. The macros are extracted from the template during parsing and never change thereafter.
+ *
+ * @author emcmanus@google.com (Éamonn McManus)
+ */
+interface EvaluationContext {
+  Object getVar(String var);
+
+  boolean varIsDefined(String var);
+
+  /**
+   * Sets the given variable to the given value.
+   *
+   * @return a Runnable that will restore the variable to the value it had before. If the variable
+   *     was undefined before this method was executed, the Runnable will make it undefined again.
+   *     This allows us to restore the state of {@code $x} after {@code #foreach ($x in ...)}.
+   */
+  Runnable setVar(final String var, Object value);
+
+  class PlainEvaluationContext implements EvaluationContext {
+    private final Map<String, Object> vars;
+
+    PlainEvaluationContext(Map<String, ?> vars) {
+      this.vars = new TreeMap<String, Object>(vars);
+    }
+
+    @Override
+    public Object getVar(String var) {
+      return vars.get(var);
+    }
+
+    @Override
+    public boolean varIsDefined(String var) {
+      return vars.containsKey(var);
+    }
+
+    @Override
+    public Runnable setVar(final String var, Object value) {
+      Runnable undo;
+      if (vars.containsKey(var)) {
+        final Object oldValue = vars.get(var);
+        undo = new Runnable() {
+          @Override public void run() {
+            vars.put(var, oldValue);
+          }
+        };
+      } else {
+        undo = new Runnable() {
+          @Override public void run() {
+            vars.remove(var);
+          }
+        };
+      }
+      vars.put(var, value);
+      return undo;
+    }
+  }
+}