presto-thrift/src/main/java/io/prestosql/plugin/thrift/ThriftTableMetadata.java/ThriftTableMetadata/deepImmutableCopy
class ThriftTableMetadata:    private static Set<Set<String>> deepImmutableCopy(List<Set<String>> indexableKeys)
    {
        return indexableKeys.stream().map(ImmutableSet::copyOf).collect(toImmutableSet());
    }