input: Write a Python function to book and cancel tickets, ensuring the ticket availability updates correctly.
  assert book_ticket("Alice", "VIP", 100) is None
  assert book_ticket("Bob", "Regular", 50) is None
  assert tickets["VIP"]["availability"] == 8
  assert tickets["VIP"]["ticket_holders"] == ["Alice"]
  assert tickets["Regular"]["availability"] == 9
  assert tickets["Regular"]["ticket_holders"] == ["Bob"]
  assert cancel_ticket("Alice", "VIP") is None
  assert tickets["VIP"]["availability"] == 9
  assert tickets["VIP"]["ticket_holders"] == []
output: tickets = {
      "VIP": {"availability": 9, "ticket_holders": []},
      "Regular": {"availability": 10, "ticket_holders": []}
  }
  
  def book_ticket(name, ticket_type, quantity):
      if ticket_type in tickets and tickets[ticket_type]["availability"] >= quantity:
          tickets[ticket_type]["availability"] -= quantity
          tickets[ticket_type]["ticket_holders"].append(name)
  
  def cancel_ticket(name, ticket_type):
      if ticket_type in tickets and name in tickets[ticket_type]["ticket_holders"]:
          tickets[ticket_type]["availability"] += 1
          tickets[ticket_type]["ticket_holders"].remove(name)