Most of us know that, being deterministic, computers cannot generate true [random numbers](http://en.wikipedia.org/wiki/Pseudorandom_number_generator).

However, let's say you have a box which generates truly random binary numbers, but is biased:  it's more likely to generate either a `1` or a `0`, but you don't know the exact probabilities, or even which is more likely *(both probabilities are > 0 and sum to 1, obviously)*

Can you use this box to create a unbiased random generator of binary numbers?