Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
import static java.util.concurrent.TimeUnit.SECONDS;
import com.google.common.testing.ClassSanityTester;
import com.google.common.util.concurrent.FuturesTest.ExecutorSpy;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import junit.framework.AssertionFailedError;
import junit.framework.TestCase;
/**
 * Unit tests for {@link JdkFutureAdapters}.
 *
 * @author Sven Mawson
 * @author Kurt Alfred Kluever
 */
public class JdkFutureAdaptersTest extends TestCase {
  private static final String DATA1 = "data";
  public void testListenInPoolThreadReturnsSameFuture() throws Exception {
    ListenableFuture<String> listenableFuture = immediateFuture(DATA1);
    assertSame(listenableFuture, listenInPoolThread(listenableFuture));
  }
  private static class SingleCallListener implements Runnable {
    private boolean expectCall = false;
    private final CountDownLatch calledCountDown = new CountDownLatch(1);
    @Override
    public void run() {
      assertTrue("Listener called before it was expected", expectCall);
      assertFalse("Listener called more than once", wasCalled());