Ok, a less elegant one then, but one that works for a reasonably simple reason. f gives 1 to 5, if it gives 5, try again, and so on, until you have a number fro

Ok, a less elegant one then, but one that works for a reasonably simple reason. f gives 1 to 5, if it gives 5, try again, and so on, until you have a number from 1 to 4. Then, do that mod 2. You have a random binary digit, that's unbiased. Now use that process to get 3 binary digits. You get a random number from 0 to 7. If the random number is 0, start again... eventually you'll get a number from 1 to 7, and all numbers have the same chances.