idea:
first use rand5() to generate a binary random function with p = 1/7 as
follows
express 1/7 in base 5
e.g. 0.abc......
(Note if x can be expressed in base 5 with finite digits, after the last
digit, we assume an infinite number of 0s are padded)
check first digit a:
if rand5() < a return 1
else if rand5() > a return 0
else proceed to the next digit
similarly generate binary random functions with p=1/6, 1/5, ..... 1/2
denote these binary random functions as rand(), N = 2,3,...,7
if rand<7>() == 1 return 0
else if rand<6>() == 1 return 1
...
else if rand<2>() == 1 return 5
else return 6
