code: func (m *MergeIterator) Close() error {
  	var rv error
  	for i := range m.itrs {
  		// close all iterators, return first error if any
  		err := m.itrs[i].Close()
  		if rv == nil {
  			rv = err
  		}
  	}
  	return rv
  }
docs: // Close will attempt to close all the underlying Iterators.  If any errors
  // are encountered, the first will be returned.