iterator primegen(limit: int = high(BiggestInt)): int:
  """
  Generates primes < limit almost lazily by a segmented sieve of Eratosthenes.
  Examples:
  >>> primegen().take(20)
  [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]
  >>> primegen(73).toSeq()
  [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]
  """