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)
```
}
    @Override
    public boolean containsKey(@NullableDecl Object key) {
      return get(key) != null;
    }
    @Override
    @NullableDecl
    public Range<C> get(@NullableDecl Object key) {
      if (key instanceof Cut) {
        try {
          @SuppressWarnings("unchecked") 
          Cut<C> cut = (Cut<C>) key;
          if (!lowerBoundWindow.contains(cut)
              || cut.compareTo(restriction.lowerBound) < 0
              || cut.compareTo(restriction.upperBound) >= 0) {
            return null;
          } else if (cut.equals(restriction.lowerBound)) {
            
            Range<C> candidate = Maps.valueOrNull(rangesByLowerBound.floorEntry(cut));
            if (candidate != null && candidate.upperBound.compareTo(restriction.lowerBound) > 0) {
              return candidate.intersection(restriction);
            }
          } else {
            Range<C> result = rangesByLowerBound.get(cut);
            if (result != null) {
              return result.intersection(restriction);
            }
          }
        } catch (ClassCastException e) {
          return null;