@Override
  public boolean retainAll(Collection collection) {
    throw new UnsupportedOperationException(
        "Region values is not modifiable");
  }
Getter for property modifiable. Since a QRegion as a collection represents an unmodifiable
facade on the values of this region, return false.

@return Value of property modifiable.
geode-core/src/main/java/org/apache/geode/cache/query/internal/QRegion.java