code: func (gp *GasPool) SubGas(amount uint64) error {
  	if uint64(*gp) < amount {
  		return ErrGasLimitReached
  	}
  	*(*uint64)(gp) -= amount
  	return nil
  }
docs: // SubGas deducts the given amount from the pool if enough gas is
  // available and returns an error otherwise.