code: public Iterable<Integer> index() {
          return () -> new Iterator<Integer>() {
              private final Iterator<Integer> it = data.records.keySet().iterator();
  
              /** {@inheritDoc} */
              @Override
              public boolean hasNext() {
                  return it.hasNext();
              }
  
              /** {@inheritDoc} */
              @Override
              public Integer next() {
                  return it.next();
              }
  
              /** {@inheritDoc} */
              @Override
              public void remove() {
                  throw new UnsupportedOperationException("This is a read-only iterator, remove operation is not supported.");
              }
          };
      }
docs: Returns a read-only Iterable on the keys of the Dataframe.
  
  @return