Introduce _onlyArbitrator modifier for increased code readability.

[marsrobertson]

https://github.com/kleros/kleros-interaction/blob/master/contracts/standard/permission/ArbitrableAddressList.sol#L472-L490

The method is public

Later on in the body there is check: require(request.arbitrator == msg.sender);

I believe that for readability, it is a good candidate for _onlyArbitrator modifier.

Makes code easier to read, immediately you can tell.

[clesaege]

I'm not a fan of modifiers except the most basic ones. Here it would complexify the code.