(Dunno how they are sold in your country) a bottle of nitroglycerine or similar drugs, the instructions to which you know by heart, similar to Harry's preparations? Considering that the probability of you encountering a stranger who has an emergency should be higher than the p of only you having it, unless there is a common cause. In case there is a common cause, well. Bring a gun?.. (At least it is small.) A notebook seems also a useful thing to havehave, with a pencil attached.