let era_sieve n =
  (* set the upper bound of numbers to check *)
  let upper = n * n in

  (* populate a list with numbers that we can record primeness of *)
  let primes = Array.create upper true in
  (* walk the numbers between 2 and the upper bound *)
  for this_num = 2 to upper do
      (* next, just iterate through the sieve *)
      (* yes, this will hit the same numbers bunches of times, but
       * this isn't about efficiency... *)
      for iterator = this_num to upper do
        let multiple = 1 in
        let next_nonprime = iterator * iterator + multiple * iterator in
          while next_nonprime < upper do
            (* well, all these numbers are certainly not prime! *)
            primes.( next_nonprime ) <- false ;
            multiple := !multiple + 1 ;
          done
      done
  done