input: Write a function to generate a unique 6 digit number.
  assert generate_number()==10000
  assert generate_number()==10001
  assert generate_number()==10002
  assert generate_number()==10003
  assert generate_number()==10004
output: def generate_number():
      generate_number.counter = getattr(generate_number, 'counter', 9999) + 1
      return generate_number.counter