Enumerate the primes using only unary arithmetic

primes.py

def is_prime(n):
  if n in ('', '1'):
    return False

  d = '11'
  while difference(n, d):
    if is_divisible(n, d):
      return False
    d += '1'
  return True

def is_divisible(a, b):
  while not is_greater(b, a):
    a = difference(a, b)
  return a == ''

def difference(a, b):
  if is_greater(b, a):
    raise Exception('second operand was greater')

  while a and b:
    a = a[1:]
    b = b[1:]
  return a

def is_greater(a, b):
  while a and b:
    a = a[1:]
    b = b[1:]
  return a != ''

n = ''
while True:
  if is_prime(n):
    print n
  n += '1'