code: func (c *ChainConfig) GasTable(num *big.Int) GasTable {
  	if num == nil {
  		return GasTableHomestead
  	}
  	switch {
  	case c.IsConstantinople(num):
  		return GasTableConstantinople
  	case c.IsEIP158(num):
  		return GasTableEIP158
  	case c.IsEIP150(num):
  		return GasTableEIP150
  	default:
  		return GasTableHomestead
  	}
  }
docs: // GasTable returns the gas table corresponding to the current phase (homestead or homestead reprice).
  //
  // The returned GasTable's fields shouldn't, under any circumstances, be changed.