input: Write a python function to add a passenger to a taxi only if the capacity allows.
  assert taxi.add_passenger("John") == True
  assert taxi.add_passenger("Alice") == True
  assert taxi.add_passenger("Bob") == False
output: def add_passenger(self, passenger_name):
      if len(self.passengers) < self.capacity:
          self.passengers.append(passenger_name)
          return True
      return False