code: func (b *Backoffer) Clone() *Backoffer {
  	return &Backoffer{
  		ctx:        b.ctx,
  		maxSleep:   b.maxSleep,
  		totalSleep: b.totalSleep,
  		errors:     b.errors,
  		vars:       b.vars,
  	}
  }
docs: // Clone creates a new Backoffer which keeps current Backoffer's sleep time and errors, and shares
  // current Backoffer's context.