*
   * @param deque the deque to be wrapped in a synchronized view
   * @return a synchronized view of the specified deque
   * @since 15.0
   */
  public static <E extends @Nullable Object> Deque<E> synchronizedDeque(Deque<E> deque) {
    return Synchronized.deque(deque, null);
  }

  }

  static <E extends @Nullable Object> Deque<E> deque(Deque<E> deque, @CheckForNull Object mutex) {
    return new SynchronizedDeque<>(deque, mutex);
  }

  static final class SynchronizedDeque<E extends @Nullable Object> extends SynchronizedQueue<E>
      implements Deque<E> {

    SynchronizedDeque(Deque<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Iterator;
import java.util.Queue;
import javax.annotation.CheckForNull;

/**
 * Views elements of a type {@code T} as nodes in a tree, and provides methods to traverse the trees

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Closeable;
import java.io.IOException;
import java.lang.reflect.Method;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.logging.Level;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**

import java.nio.ShortBuffer;
import java.nio.charset.Charset;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Currency;
import java.util.Deque;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.Locale;
import java.util.Map;
import java.util.NavigableMap;
import java.util.NavigableSet;

import java.nio.ShortBuffer;
import java.nio.charset.Charset;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Currency;
import java.util.Deque;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.Locale;
import java.util.Map;
import java.util.NavigableMap;
import java.util.NavigableSet;

     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */

    @CheckForNull private Iterator<? extends Iterator<? extends T>> topMetaIterator;

    // Only becomes nonnull if we encounter nested concatenations.
    @CheckForNull private Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;