Bitcoin example removed
If you don't mind, i have removed the "BTC" example because it is not part of the ISO 4712 standard and therefore it doesn't fit to the definition of this tag. According to taginfo, currency:BTC=* is used exactly zero times.
I suggest if anyone wants to allow non-ISO-4712 currency codes we should document each of them explicitly in this wiki page. So far I haven't seen any need for this. In case this need comes up some day, feel free to drop a message here. --Wuzzy 19:24, 28 November 2012 (UTC)