import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.Maps.newTreeMap;
import static java.util.Collections.singletonMap;
import static java.util.Collections.unmodifiableSortedMap;

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.SortedMap;
import java.util.TreeMap;

                SortedMap<String, String> map = populate(new TreeMap<String, String>(), entries);
                return Collections.unmodifiableSortedMap(map);
              }
            })
        .named("unmodifiableSortedMap/TreeMap, natural")
        .withFeatures(
            MapFeature.ALLOWS_NULL_VALUES,
            CollectionFeature.KNOWN_ORDER,
            CollectionFeature.SERIALIZABLE,

                SortedMap<String, String> map = populate(new TreeMap<String, String>(), entries);
                return Collections.unmodifiableSortedMap(map);
              }
            })
        .named("unmodifiableSortedMap/TreeMap, natural")
        .withFeatures(
            MapFeature.ALLOWS_NULL_VALUES,
            CollectionFeature.KNOWN_ORDER,
            CollectionFeature.SERIALIZABLE,

    /*
     * It's not ? extends R, because it's technically not covariant in R. Specifically,
     * table.rowMap().comparator() could return a comparator that only works for the ? extends R.
     * Collections.unmodifiableSortedMap makes the same distinction.
     */
    return new UnmodifiableRowSortedMap<>(table);
  }

  private static final class UnmodifiableRowSortedMap<

    /*
     * It's not ? extends R, because it's technically not covariant in R. Specifically,
     * table.rowMap().comparator() could return a comparator that only works for the ? extends R.
     * Collections.unmodifiableSortedMap makes the same distinction.
     */
    return new UnmodifiableRowSortedMap<>(table);
  }

  private static final class UnmodifiableRowSortedMap<

      comparator = (Comparator<? super K>) NATURAL_ORDER;
    }
    if (map instanceof ImmutableSortedMap) {
      // TODO(kevinb): Prove that this cast is safe, even though
      // Collections.unmodifiableSortedMap requires the same key type.
      @SuppressWarnings("unchecked")
      ImmutableSortedMap<K, V> kvMap = (ImmutableSortedMap<K, V>) map;
      if (!kvMap.isPartialView()) {
        return kvMap;
      }
    }

      comparator = (Comparator<? super K>) NATURAL_ORDER;
    }
    if (map instanceof ImmutableSortedMap) {
      // TODO(kevinb): Prove that this cast is safe, even though
      // Collections.unmodifiableSortedMap requires the same key type.
      @SuppressWarnings("unchecked")
      ImmutableSortedMap<K, V> kvMap = (ImmutableSortedMap<K, V>) map;
      if (!kvMap.isPartialView()) {
        return kvMap;
      }
    }

    }
  }

  private static <K extends @Nullable Object, V extends @Nullable Object> Map<K, V> unmodifiableMap(
      Map<K, ? extends V> map) {
    if (map instanceof SortedMap) {
      return Collections.unmodifiableSortedMap((SortedMap<K, ? extends V>) map);
    } else {
      return Collections.unmodifiableMap(map);
    }
  }

  static class MapDifferenceImpl<K extends @Nullable Object, V extends @Nullable Object>

    }
  }

  private static <K extends @Nullable Object, V extends @Nullable Object> Map<K, V> unmodifiableMap(
      Map<K, ? extends V> map) {
    if (map instanceof SortedMap) {
      return Collections.unmodifiableSortedMap((SortedMap<K, ? extends V>) map);
    } else {
      return Collections.unmodifiableMap(map);
    }
  }

  static class MapDifferenceImpl<K extends @Nullable Object, V extends @Nullable Object>