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)
```
private static final class ComplementRangesByLowerBound<C extends Comparable<?>>
      extends AbstractNavigableMap<Cut<C>, Range<C>> {
    private final NavigableMap<Cut<C>, Range<C>> positiveRangesByLowerBound;
    private final NavigableMap<Cut<C>, Range<C>> positiveRangesByUpperBound;
    /**
     * complementLowerBoundWindow represents the headMap/subMap/tailMap view of the entire
     * "complement ranges by lower bound" map; it's a constraint on the *keys*, and does not affect
     * the values.
     */
    private final Range<Cut<C>> complementLowerBoundWindow;
    ComplementRangesByLowerBound(NavigableMap<Cut<C>, Range<C>> positiveRangesByLowerBound) {
      this(positiveRangesByLowerBound, Range.<Cut<C>>all());
    }
    private ComplementRangesByLowerBound(
        NavigableMap<Cut<C>, Range<C>> positiveRangesByLowerBound, Range<Cut<C>> window) {
      this.positiveRangesByLowerBound = positiveRangesByLowerBound;
      this.positiveRangesByUpperBound = new RangesByUpperBound<C>(positiveRangesByLowerBound);
      this.complementLowerBoundWindow = window;
    }
    private NavigableMap<Cut<C>, Range<C>> subMap(Range<Cut<C>> subWindow) {
      if (!complementLowerBoundWindow.isConnected(subWindow)) {
        return ImmutableSortedMap.of();
      } else {
        subWindow = subWindow.intersection(complementLowerBoundWindow);
        return new ComplementRangesByLowerBound<C>(positiveRangesByLowerBound, subWindow);
      }
    }
    @Override
    public NavigableMap<Cut<C>, Range<C>> subMap(
        Cut<C> fromKey, boolean fromInclusive, Cut<C> toKey, boolean toInclusive) {