code: func munmap(db *DB) error {
  	// Ignore the unmap if we have no mapped data.
  	if db.dataref == nil {
  		return nil
  	}
  
  	// Unmap using the original byte slice.
  	err := unix.Munmap(db.dataref)
  	db.dataref = nil
  	db.data = nil
  	db.datasz = 0
  	return err
  }
docs: // munmap unmaps a DB's data file from memory.