Niemand
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Mix rum with cola? Alex, your friends are either real vandals or masochists.
In general, you shouldn't mix only beer. Never and with nothing. And also, maybe champagne. Everything else is fine. With water (not tap water or soda) or juice.
And dark rum is best enjoyed as a Hemingway Daiquiri. Also, after a shot of dark rum, dark beer goes well. Don't try white rum – it's awful. Worse than vodka.