code: func FileDescOk(fd FileDesc) bool {
  	switch fd.Type {
  	case TypeManifest:
  	case TypeJournal:
  	case TypeTable:
  	case TypeTemp:
  	default:
  		return false
  	}
  	return fd.Num >= 0
  }
docs: // FileDescOk returns true if fd is a valid 'file descriptor'.