func: function topUpGas(uint _amount) external isNotZero(_amount) {
          
          require(_isOwner() || _isController(msg.sender), "sender is neither an owner nor a controller");
          
          _updateTopUpAvailable();
          
          require(_topUpAvailable != 0, "available top up limit cannot be zero");
          
          require(_amount <= _topUpAvailable, "available top up limit less than amount passed in");
          
          
          _topUpAvailable = _topUpAvailable.sub(_amount);
          owner().transfer(_amount);
          
          emit ToppedUpGas(tx.origin, owner(), _amount);
      }