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)
```
Range<C> positiveRange = positiveItr.next();
            negativeRange = Range.create(nextComplementRangeLowerBound, positiveRange.lowerBound);
            nextComplementRangeLowerBound = positiveRange.upperBound;
          } else {
            negativeRange = Range.create(nextComplementRangeLowerBound, Cut.<C>aboveAll());
            nextComplementRangeLowerBound = Cut.aboveAll();
          }
          return Maps.immutableEntry(negativeRange.lowerBound, negativeRange);
        }
      };
    }
    @Override
    Iterator<Entry<Cut<C>, Range<C>>> descendingEntryIterator() {
      /*
       * firstComplementRangeUpperBound is the upper bound of the last complement range with lower
       * bound inside complementLowerBoundWindow.
       *
       * positiveItr starts at the first positive range with upper bound less than
       * firstComplementRangeUpperBound. (Positive range upper bounds correspond to complement range
       * lower bounds.)
       */
      Cut<C> startingPoint =
          complementLowerBoundWindow.hasUpperBound()
              ? complementLowerBoundWindow.upperEndpoint()
              : Cut.<C>aboveAll();
      boolean inclusive =
          complementLowerBoundWindow.hasUpperBound()
              && complementLowerBoundWindow.upperBoundType() == BoundType.CLOSED;
      final PeekingIterator<Range<C>> positiveItr =
          Iterators.peekingIterator(