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)
```
positiveRanges =
            positiveRangesByUpperBound
                .tailMap(
                    complementLowerBoundWindow.lowerEndpoint(),
                    complementLowerBoundWindow.lowerBoundType() == BoundType.CLOSED)
                .values();
      } else {
        positiveRanges = positiveRangesByUpperBound.values();
      }
      final PeekingIterator<Range<C>> positiveItr =
          Iterators.peekingIterator(positiveRanges.iterator());
      final Cut<C> firstComplementRangeLowerBound;
      if (complementLowerBoundWindow.contains(Cut.<C>belowAll())
          && (!positiveItr.hasNext() || positiveItr.peek().lowerBound != Cut.<C>belowAll())) {
        firstComplementRangeLowerBound = Cut.belowAll();
      } else if (positiveItr.hasNext()) {
        firstComplementRangeLowerBound = positiveItr.next().upperBound;
      } else {
        return Iterators.emptyIterator();
      }
      return new AbstractIterator<Entry<Cut<C>, Range<C>>>() {
        Cut<C> nextComplementRangeLowerBound = firstComplementRangeLowerBound;
        @Override
        protected Entry<Cut<C>, Range<C>> computeNext() {
          if (complementLowerBoundWindow.upperBound.isLessThan(nextComplementRangeLowerBound)
              || nextComplementRangeLowerBound == Cut.<C>aboveAll()) {
            return endOfData();
          }
          Range<C> negativeRange;
          if (positiveItr.hasNext()) {