class Prime::EratosthenesSieve

def indices(n)

for an odd number +n+, returns (i, j, k) such that @tables[i][j][k] represents primarity of the number 
def indices(n)
  #   binary digits of n: |0|1|2|3|4|5|6|7|8|9|10|11|....
  #   indices:            |-|    k  |  j  |     i
  # because of NUMS_PER_ENTRY, NUMS_PER_TABLE
  k = (n & 0b00011111) >> 1
  j = (n & 0b11100000) >> 5
  i = n >> 8
  return i, j, k
end