Callback is often automated through the use of web callback or mobile phone applications. First a function is defined with a parameter intended for callback&colon; . A typical use of a function object is in writing callback functions. It is possible to use function objects in situations other than as callback functions. Then a function that can be used as a callback to is defined, . In this example, is invoked twice, once with as a callback and once with . Cache consistency is maintained by callback mechanism. Here is a very trivial example of the use of a callback in Python. The software was previously called just "PhoneGap", then "Apache Callback". A callback is a reference to a previous joke. The line between a callback and simple continuity can be ambiguous. The project's original name, Apache Callback, was viewed as too generic. In this primitive example, the use of a callback is primarily a demonstration of principle. The episode also features several callback references to previous episodes. Ricky Wong was instrumental to the introduction of callback IDD service to Hong Kong.