Exponential sampling is wrong

Formula (6.67, 6.68). Statement that `-log(1-u)` is equivalent to `-log(u)` is true iff `u` is U(0,1) in the (0,1) range. In reality Python U(0,1) as well as C/C++ U(0,1) are defined in the interval [0,1). Thus, `-log(1-u)` will always work while `-log(u)` sometimes will produce FP exceptions when 0 is sampled