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)
```
TreeRangeSet<C> result = create();
    result.addAll(ranges);
    return result;
  }
  private TreeRangeSet(NavigableMap<Cut<C>, Range<C>> rangesByLowerCut) {
    this.rangesByLowerBound = rangesByLowerCut;
  }
  @NullableDecl private transient Set<Range<C>> asRanges;
  @NullableDecl private transient Set<Range<C>> asDescendingSetOfRanges;
  @Override
  public Set<Range<C>> asRanges() {
    Set<Range<C>> result = asRanges;
    return (result == null) ? asRanges = new AsRanges(rangesByLowerBound.values()) : result;
  }
  @Override
  public Set<Range<C>> asDescendingSetOfRanges() {
    Set<Range<C>> result = asDescendingSetOfRanges;
    return (result == null)
        ? asDescendingSetOfRanges = new AsRanges(rangesByLowerBound.descendingMap().values())
        : result;
  }
  final class AsRanges extends ForwardingCollection<Range<C>> implements Set<Range<C>> {
    final Collection<Range<C>> delegate;
    AsRanges(Collection<Range<C>> delegate) {
      this.delegate = delegate;
    }
    @Override
    protected Collection<Range<C>> delegate() {
      return delegate;
    }