final List<Tree> children;

    public Tree(char value, Tree... children) {
      super(value);
      this.children = Arrays.asList(children);
    }
  }

  private static final TreeTraverser<Tree> ADAPTER =
      new TreeTraverser<Tree>() {
        @Override
        public Iterable<Tree> children(Tree node) {
          return node.children;
        }
      };

    checkNotNull(nodeToChildrenFunction);
    return new TreeTraverser<T>() {
      @Override
      public Iterable<T> children(T root) {
        return nodeToChildrenFunction.apply(root);
      }
    };
  }

  /** Returns the children of the specified node. Must not contain null. */
  public abstract Iterable<T> children(T root);

  /**

    final List<Tree> children;

    public Tree(char value, Tree... children) {
      super(value);
      this.children = Arrays.asList(children);
    }
  }

  private static final TreeTraverser<Tree> ADAPTER =
      new TreeTraverser<Tree>() {
        @Override
        public Iterable<Tree> children(Tree node) {
          return node.children;
        }
      };

  <build>
    <resources>
      <resource>
        <directory>..</directory>
        <includes>
          <include>LICENSE</include> <!-- copied from the parent pom because I couldn't figure out a way to make combine.children="append" work -->
          <include>proguard/*</include>
        </includes>
        <targetPath>META-INF</targetPath>
      </resource>
    </resources>
    <plugins>
      <plugin>

                   JDK6-like APIs.

                   So instead, we use javac9, which doesn't lead to this error.
              -->
              <compilerArgs combine.children="append">
                <arg>-J-Xbootclasspath/p:${settings.localRepository}/com/google/errorprone/javac/${javac.version}/javac-${javac.version}.jar</arg>
              </compilerArgs>
            </configuration>

  <build>
    <resources>
      <resource>
        <directory>..</directory>
        <includes>
          <include>LICENSE</include> <!-- copied from the parent pom because I couldn't figure out a way to make combine.children="append" work -->
          <include>proguard/*</include>
        </includes>
        <targetPath>META-INF</targetPath>
      </resource>
    </resources>
    <plugins>
      <plugin>

        int index = i++;
        future.addListener(
            () -> {
              try {
                if (future.isCancelled()) {
                  // Clear futures prior to cancelling children. This sets our own state but lets
                  // the input futures keep running, as some of them may be used elsewhere.
                  futures = null;
                  cancel(false);
                } else {