code: func useGasNegative(gasLeft *uint64, gasToUse uint64, err errors.Sink) {
  	if *gasLeft >= gasToUse {
  		*gasLeft -= gasToUse
  	} else {
  		err.PushError(errors.ErrorCodeInsufficientGas)
  	}
  }
docs: // Try to deduct gasToUse from gasLeft.  If ok return false, otherwise
  // set err and return true.