code: func (s *Schedule) Validate(c *validation.Context) {
  	if s.GetAmount() < 0 {
  		c.Errorf("amount must be non-negative")
  	}
  	c.Enter("length")
  	s.GetLength().Validate(c)
  	switch n, err := s.Length.ToSeconds(); {
  	case err != nil:
  		c.Errorf("%s", err)
  	case n == 0:
  		c.Errorf("duration or seconds is required")
  	}
  	c.Exit()
  	c.Enter("start")
  	s.GetStart().Validate(c)
  	c.Exit()
  }
docs: // Validate validates this schedule.