code: func (c *ChainConfig) Rules(num *big.Int) Rules {
  	chainID := c.ChainID
  	if chainID == nil {
  		chainID = new(big.Int)
  	}
  	return Rules{
  		ChainID:          new(big.Int).Set(chainID),
  		IsHomestead:      c.IsHomestead(num),
  		IsEIP150:         c.IsEIP150(num),
  		IsEIP155:         c.IsEIP155(num),
  		IsEIP158:         c.IsEIP158(num),
  		IsByzantium:      c.IsByzantium(num),
  		IsConstantinople: c.IsConstantinople(num),
  		IsPetersburg:     c.IsPetersburg(num),
  	}
  }
docs: // Rules ensures c's ChainID is not nil.