code: func (file *GridFile) SetId(id interface{}) {
  	file.assertMode(gfsWriting)
  	file.m.Lock()
  	file.doc.Id = id
  	file.m.Unlock()
  }
docs: // SetId changes the current file Id.
  //
  // It is a runtime error to call this function once the file has started
  // being written to, or when the file is not open for writing.