code: func (self *StateDB) SubRefund(gas uint64) {
  	self.journal.append(refundChange{prev: self.refund})
  	if gas > self.refund {
  		panic("Refund counter below zero")
  	}
  	self.refund -= gas
  }
docs: // SubRefund removes gas from the refund counter.
  // This method will panic if the refund counter goes below zero