[competentone]

For a mugging threat it's probably wiser to carry a throw-away wallet. Have a few one dollar bills sticking partially out of it.

If someone pulls a gun or knife on you and says, "Give me your money!" Pull out the wallet, throw it in one direction while you run in the opposite, yelling, "Fire, fire, fire!"

(People are more likely to pay attention to a call of "fire" since it can potentially affect them, than they are to respond to someone yelling "help" or "robbery.")