Before revision: For instance, if there is a coin flip that on heads will create one observer, while on tails they will create two, then we have three possible observers (1st observer on heads, 1st on tails, 2nd on tails), each existing with probability 0.5, so SIA assigns 1/3 probability to each.
After revision: For instance, if there is a coin flip that on heads will create one observer, while on tails it will create two, then we have three possible observers (1st observer on heads, 1st on tails, 2nd on tails), each existing with probability 0.5, so SIA assigns 1/3 probability to each.