在主引擎中相关方法都没有设置的时候，为什么可以将OmsEngine子类的方法直接设置为主引擎的方法？
def add_function(self) -> None:
"""Add query function to main engine."""
self.main_engine.get_tick = self.get_tick
self.main_engine.get_order = self.get_order
self.main_engine.get_trade = self.get_trade
self.main_engine.get_position = self.get_position
self.main_engine.get_account = self.get_account
self.main_engine.get_contract = self.get_contract
self.main_engine.get_quote = self.get_quote
self.main_engine.get_all_ticks = self.get_all_ticks
self.main_engine.get_all_orders = self.get_all_orders
self.main_engine.get_all_trades = self.get_all_trades
self.main_engine.get_all_positions = self.get_all_positions
self.main_engine.get_all_accounts = self.get_all_accounts
self.main_engine.get_all_contracts = self.get_all_contracts
self.main_engine.get_all_quotes = self.get_all_quotes
self.main_engine.get_all_active_orders = self.get_all_active_orders
self.main_engine.get_all_active_quotes = self.get_all_active_quotes
